ARM: tegra: enable gpio-keys on Dalmore

Dalmore have the keys mounted on board which are connected
to different pins of Tegra.

Add the keys entry in DTS file to enable key functionality.
This will enable KEY_POWER, KEY_HOME, KEY_VOLUMEUP and
KEY_VOLUMEDOWN.

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