Make cache coherency attributes definitions available in machine/vm.h on MIPS.

Move definitions from cpuregs.h into the cca.h, and include cca.h into vm.h.
This is required to make MIPS MD memattr definitions usable in userspace.

Sponsored by: The FreeBSD Foundation, Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D15583