dt-bindings: gpio: dwapb: Add ngpios property support

It's redundant to have a vendor-specific property describing a number of
GPIOS while there is a generic one. Let's mark the former one as
deprecated and define the "ngpios" property supported with constraints
of being within [1; 32] range.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200730152808.2955-2-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>