pinctrl: Protect the pinctrl_maps in pinctrl_register_map

The pinctrl_maps_mutex didn't take affect in pinctrl_register_map
sometimes. Now the change make it to protect the pinctrl_maps list
operation all the time.

CRs-Fixed: 649599
Change-Id: I5e4ce53b84375e4b1cf08f362bb26cbbe8acc07e
Signed-off-by: Pan Fang <fangpan@codeaurora.org>