As part of ARMv8M, we need to add support for the PMSAv8 MPU
architecture.
PMSAv8 differs from PMSAv7 both in register/data layout (for instance
using base and limit registers rather than base and size) and also in
behaviour (for example it does not have subregions); rather than
trying to wedge it into the existing PMSAv7 code and data structures,
we define separate ones.
This commit adds the data structures which hold the state for a
PMSAv8 MPU and the register interface to it. The implementation of
the MPU behaviour will be added in a subsequent commit.
Backports commit 0e1a46bbd2d6c39614b87f4e88ea305acce8a35f from qemu
1acd9efdc2