sc_pll: Mask out LOCK_STATUS bit

Mask out the LOCK_STATUS bit when writing to the control_reg1. This
should not make a difference on the HW (as writes of 0 to this bit
should be ignored), but it does make a difference with the unittest
where this bit can be written to. In this case, the SW will set this bit
to 0 and then spin on it and timeout, causing the tests to fail.

Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>