Added driver to control the external power output

This PR adds support to control the external power output from controllers like nice!nano, nRFMicro etc

I have implemented based on my understanding of Pete suggestion on this feature.

Testing done:

    Tested by enabling and disabling the ext_power from application and verified
    Verified the application does not crash with boards that does not have ext_power support
    Note:
    I did not test this in nice!nano since I don't have the boards. Will get help from others once the behavior PR is up

Next Steps:

    Create a behavior PR to control enable/disable ext_power

1 files changed, 104 insertions, 0 deletions

diff --git a/app/include/drivers/ext_power.h b/app/include/drivers/ext_power.h
new file mode 100644
index 0000000..6c1923e
--- /dev/null
+++ b/app/include/drivers/ext_power.h
@@ -0,0 +1,104 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/types.h>
+#include <stddef.h>
+#include <device.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @cond INTERNAL_HIDDEN
+ *
+ * Behavior driver API definition and system call entry points.
+ *
+ * (Internal use only.)
+ */
+
+typedef int (*ext_power_enable_t)(struct device *dev);
+typedef int (*ext_power_disable_t)(struct device *dev);
+typedef int (*ext_power_get_t)(struct device *dev);
+
+__subsystem struct ext_power_api {
+    ext_power_enable_t enable;
+    ext_power_disable_t disable;
+    ext_power_get_t get;
+};
+/**
+ * @endcond
+ */
+
+/**
+ * @brief Enable the external power output
+ * @param dev Pointer to the device structure for the driver instance.
+ *
+ * @retval 0 If successful.
+ * @retval Negative errno code if failure.
+ */
+__syscall int ext_power_enable(struct device *dev);
+
+static inline int z_impl_ext_power_enable(struct device *dev) {
+    const struct ext_power_api *api = (const struct ext_power_api *)dev->driver_api;
+
+    if (api->enable == NULL) {
+        return -ENOTSUP;
+    }
+
+    return api->enable(dev);
+}
+
+/**
+ * @brief Disable the external power output
+ * @param dev Pointer to the device structure for the driver instance.
+ *
+ * @retval 0 If successful.
+ * @retval Negative errno code if failure.
+ */
+__syscall int ext_power_disable(struct device *dev);
+
+static inline int z_impl_ext_power_disable(struct device *dev) {
+    const struct ext_power_api *api = (const struct ext_power_api *)dev->driver_api;
+
+    if (api->disable == NULL) {
+        return -ENOTSUP;
+    }
+
+    return api->disable(dev);
+}
+
+/**
+ * @brief Get the current status of the external power output
+ * @param dev Pointer to the device structure for the driver instance.
+ *
+ * @retval 0 If ext power is disabled.
+ * @retval 1 if ext power is enabled.
+ * @retval Negative errno code if failure.
+ */
+__syscall int ext_power_get(struct device *dev);
+
+static inline int z_impl_ext_power_get(struct device *dev) {
+    const struct ext_power_api *api = (const struct ext_power_api *)dev->driver_api;
+
+    if (api->get == NULL) {
+        return -ENOTSUP;
+    }
+
+    return api->get(dev);
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+/**
+ * @}
+ */
+
+#include <syscalls/ext_power.h>