dt-bindings: serio: add Arm PL050 DT schema

The Arm PL050 "Keyboard/Mouse Interface" is an Arm system IP providing a
PS/2 compatible serial interface.

Add a simple DT schema binding, based on the TRM[1], the existing DTs and
the Linux driver.

[1] https://developer.arm.com/documentation/ddi0143/latest

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-6-andre.przywara@arm.com