ARM: tegra: whistler: enable keyboard in DT

Enable Tegra based keyboard controller and populate the key mapping
for Whistler.

With this patch, HOME, BACK, POWER and MENU keys will work.
Still other keys which are in ROW3 and ROW4 will not work as it
conflicts with KBC pins on SDIO2 pinmux.

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