powerpc/mm: Add MMU features for TLB reservation & Paired MAS registers

Support for TLB reservation (or TLB Write Conditional) and Paired MAS
registers are optional for a processor implementation so we handle
them via MMU feature sections.

We currently only used paired MAS registers to access the full RPN + perm
bits that are kept in MAS7||MAS3.  We assume that if an implementation has
hardware page table at this time it also implements in TLB reservations.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>