mx5: Fix clock gate values

The clock gate values are 2-bit bit-fields. Hence, setting or clearing only one
of these bits like what was done is wrong and can lead to unpredictable behavior
depending on the original value of these bit-fields.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>