UPSTREAM: 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>
(cherry picked from commit 8851b9f1625ce0858e9b1bb0ae4a57d4b43178b1)

BUG=None
TEST=Verified kernel built and symbols were referenced

Change-Id: I38eb8d31f5cdd7aac47c98531df151fc0643359c
Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-on: https://chromium-review.googlesource.com/180071
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Commit-Queue: Andrew Bresticker <abrestic@chromium.org>