powerpc/476: Add isync after loading mmu and debug spr's

476 requires an isync after loading MMU and debug related SPR's.  Some of
these are in performance-critical paths and may need to be optimized, but
initially, we're playing it safe.

Signed-off-by: Torez Smith <lnxtorez@linux.vnet.ibm.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>