reset: protect struct reset_controller_dev with its own mutex

reset: protect struct reset_controller_dev with its own mutex

Currently we use a single, global mutex - misleadingly names
reset_list_mutex - to protect the global list of reset devices,
per-controller list of reset control handles and also internal fields of
struct reset_control. Locking can be made a lot more fine-grained if we
use a separate mutex for serializing operations on the list AND
accessing the reset controller device.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>