Input: gpio-keys-polled - don't use unit-address with button nodes

The change corrects an example in device tree documentation section,
because button/key device nodes don't contain reg property there is
no need to declare unit-address, and address and size cell properties
are also redundant.

At the moment a compilation of the hypothetical original example
should produce W=1 level warnings, and it is better to stop spreading
misusage of the polled gpio keys device tree binding through this
example.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>