dt-bindings: gpio: Convert snps,dw-apb-gpio to DT schema

dt-bindings: gpio: Convert snps,dw-apb-gpio to DT schema

Modern device tree bindings are supposed to be created as YAML-files
in accordance with DT schema. This commit replaces Synopsys DW GPIO
legacy bare text binding with YAML file. As before the binding file
states that the corresponding dts node is supposed to be compatible
with generic DW I2C controller indicated by the "snps,dw-apb-gpio"
compatible string and to provide a mandatory registers memory range.
It may also have an optional clock and reset phandle references.

There must be specified at least one subnode with
"snps,dw-apb-gpio-port" compatible string indicating the GPIO port,
which would actually export the GPIO controller functionality. Such
nodes should have traditional GPIO controller properties together
with optional interrupt-controller attributes if the corresponding
controller was synthesized to detect and report the input values
change to the parental IRQ controller.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200323195401.30338-2-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>