Reworked FSL Book-E TLB macros to be more readable

The old macros made it difficult to know what WIMGE and perm bits
were set for a TLB entry.  Actually use the bit masks for these items
since they are only a single bit.

Also moved the macros into mmu.h out of e500.h since they aren't specific
to e500.

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