Convert the gpio-keys and gpio-keys-polled bindings to a DT schema. As
both bindings are almost the same, combine them into a single schema.

The binding said 'interrupts' was required, but testing on dts files
showed that it isn't required.

'linux,input-value' was only documented for gpio-keys-polled, but there
doesn't seem to be any reason for it to be specific to that. It can work
for gpio-keys too if the GPIO line(s) are available.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>