mfd: Implement bus_lock/bus_sync_unlock twl4030-irq methods

For doing that, drop the locking and change that
to a mutex.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>