ACPI / LPSS: Rename pwm_backlight pwm-lookup to pwm_soc_backlight

At least Bay Trail (BYT) and Cherry Trail (CHT) devices can use 1 of 2
different PWM controllers for controlling the LCD's backlight brightness.
Either the one integrated into the PMIC or the one integrated into the
SoC (the 1st LPSS PWM controller).

So far in the LPSS code on BYT we have skipped registering the LPSS PWM
controller "pwm_backlight" lookup entry when a Crystal Cove PMIC is
present, assuming that in this case the PMIC PWM controller will be used.

On CHT we have been relying on only 1 of the 2 PWM controllers being
enabled in the DSDT at the same time; and always registered the lookup.

So far this has been working, but the correct way to determine which PWM
controller needs to be used is by checking a bit in the VBT table and
recently I've learned about 2 different BYT devices:
Point of View MOBII TAB-P800W
Acer Switch 10 SW5-012

Which use a Crystal Cove PMIC, yet the LCD is connected to the SoC/LPSS
PWM controller (and the VBT correctly indicates this), so here our old
heuristics fail.

Since only the i915 driver has access to the VBT, this commit renames
the "pwm_backlight" lookup entries for the 1st BYT/CHT LPSS PWM controller
to "pwm_soc_backlight" so that the i915 driver can do a pwm_get() for
the right controller depending on the VBT bit, instead of the i915 driver
relying on a "pwm_backlight" lookup getting registered which magically
points to the right controller.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216202906.1662893-2-hdegoede@redhat.com

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