gpio: dwapb: Add ngpios DT-property support

Indeed generic GPIO DT-schema implies that number of GPIOs should be
described by the "ngpios" property located under a GPIO-provider DT node.
In that case it's redundant to have a vendor-specific "snps,nr-gpios"
property describing the same setting. Moreover it might be errors prone.

Since commit 93d2e4322aa7 ("of: platform: Batch fwnode parsing when adding
all top level devices") the fwnode parsing is resumed after the vast
majority of the platform devices are added. Implicitly that commit
activates re-parsing of the whole device tree GPIOs-phandle properties
detected having "-gpio/-gpios" suffixes. Since the vendor-specific number
of GPIOs property is defined with "-gpios" suffix, then of_link_property()
will consider it as a suffix-property with "#gpio-cells" structure, which
obviously it doesn't have. As a result for two DW APB GPIO controllers
we'll have the next errors printed.

OF: /bus@1f059000/gpio@1f044000/gpio-port@0: could not find phandle
OF: /bus@1f059000/gpio@1f045000/gpio-port@0: could not get #gpio-cells for /opp-table
OF: /bus@1f059000/gpio@1f044000/gpio-port@0: could not find phandle
OF: /bus@1f059000/gpio@1f045000/gpio-port@0: could not get #gpio-cells for /opp-table

See, the kernel fwnode parsing procedure even tried to resolve the phandle
ID, which it thought was the opp-table DT-node, while in fact it was just
a number "32". What would happen if that magic number actually referred to
a GPIO DT-node with "#gpio-cells" property defined?..

In order to fix the problem let's mark the "snps,nr-gpios" property as
deprecated and add the generic "ngpios" property support with the same
purpose as the deprecated one. That and the errors log above shall
motivate the platform developer to convert the DW APB GPIO DT-nodes to
using the standard number of GPIOs property.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200730152808.2955-3-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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