arm: nxp: mpu: Fix region descriptor 0 attributes

Clearing fields in the region descriptor attributes doesn't always have
the expected effect of revoking permissions. In the case of bus master
supervisor mode fields (MxSM), setting to zero actually enables read,
write, and execute access.

When we reworked handling of region descriptor 0, we inadvertently
enabled execution from RAM by clearing the MxSM fields and enabling the
descriptor. This caused samples/mpu_test run to throw a usage fault
instead of an MPU-triggered bus fault.

Fix this by setting all the MxSM fields to 2'b11, which gives supervisor
mode the same access as user mode.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>