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>