gpio/omap: use flag to identify wakeup domain

In omap3, save/restore context is implemented for GPIO banks 2-6 as GPIO bank1
is in wakeup domain. Instead of identifying bank's power domain by bank id,
use 'loses_context' flag which is filled by pwrdm_can_ever_lose_context()
during dev_init.

For getting the powerdomain pointer, omap_hwmod_get_pwrdm() is used.
omap_device_get_pwrdm() could not be used as the pwrdm information needs to be
filled in pdata, whereas omap_device_get_pwrdm() could be used only after
omap_device_build() call.

Signed-off-by: Charulatha V <charu@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>