arm_mpu: reduce boot MPU regions for various soc

The PPB, Peripheral, and any other I/O regions have been
removed, the Shareable options are only important with
unlocked dual cores. Just use the background mapping,
which is RWX for supervisor and no access for user mode.

The flash region needs to be kept to indicate read-only
policy. The RAM regions need to be kept to disable execution.

Fixes #6896

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>