GPIO: OMAP: add locking around calls to _set_gpio_triggering

_set_gpio_triggering uses read-modify-write on bank registers,
lock bank->lock around all calls to it to prevent register
corruption if two cpus access gpios in the same bank at the
same time.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>