target/xtensa: implement MPU option

target/xtensa: implement MPU option

The Memory Protection Unit Option (MPU) is a combined instruction and
data memory protection unit with more protection flexibility than the
Region Protection Option or the Region Translation Option but without
any translation capability. It does no demand paging and does not
reference a memory-based page table.

Add memory protection unit option, internal state, SRs and opcodes.
Implement MPU entries dumping in dump_mmu.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>