module/mpmm: clear bits before writing to MPMMCR

When setting the MPMM threshold for a specific core, the MPMMCR value
was increased using OR operators. However, this approach is only valid
if the MPMMCR value is initially 0. Therefore, add a bit-clear
instruction before setting the MPMM threshold.

Signed-off-by: Haaris Farooq <haaris.farooq@arm.com>