ARM: tegra: use dt-binding header for key code

In place of hardcoding the key code in DTS file and comment the
key code as side notes, use the key code macro defines in the
dt-bindings/input/input.h directly.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>