gpio: davinci: Add support for system suspend/resume PM

Add support for system suspend/resume PM hooks, save the
register context of all the required gpio registers on suspend
and restore context on the resume.

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>