armv7m: add MPU configuration support

Cortex-M archs support option memory protection unit (MPU). MPU is used
to set the memory types, attributes, access permissions for different regions,
cache policies of the device.

e.g. using MPU it is possible to configure memory region as device memory
or strongly ordered, memory attributes like execute never, cache policies
like write-back or write-through.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>