ARM: tegra: add power gpio keys to DT

This adds the power gpio key to DT and enable the wakeup of the gpio key
for the device. The Seaboard and paz00 already had the power gpio key
binding and the power key of Whistler was on KBC. So these boards' device
tree didn't include in this patch.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>