Silence warning in MPU driver

The loadAndEnableConfig func is always exposed through the driver API,
even if the underlying CPU is not supported. For cases where the
function is called but its body empty, silence warnings about unused
parameters.

Change-Id: Idf93f9afc9e5c053fa3587c350abe7cc6ad5a5fa
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>