include: dt-binding: input: create a DT header defining key codes.

Many of Key device tree bindings uses the constant number as key code
which matches with kernel header key code and then comment as follows
for reference/better readability:
	linux,code = <102>; /* KEY_HOME */

Create a DT header which defines all the key code so that DT key bindings
can use it as follows:
	linux,code = <KEY_HOME>;

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>