dt-bindings: gpio: Add DW GPIO debounce clock property

Port A of the DW GPIO controller may optionally have a debounce
logic enabled if it was synthesized with that functionality enabled.
In this case a dedicated reference clock should be declared in the
node with corresponding "db" name presented in the clock-names
property.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
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>
Link: https://lore.kernel.org/r/20200323195401.30338-3-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>