arm: st_stm32: reduce boot MPU regions

The PPB and Peripheral regions have been removed,
the Shareable options are only important with unlocked
dual cores. Just use the background mapping.

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

Related to #6896 and #7877

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