[POWERPC] 8xx: platform specific mmu updates

This is just a straight port of the same done in arch/ppc
by Marcelo Tosatti. One used to be
[PATCH] ppc32 8xx: update_mmu_cache() needs unconditional tlbie,
commit eb07d964b4491d1bb5864cd3d7e7633ccdda9a53

In a nutshell, the board is nearly stuck without this, yet without any
visible failure - being just very slow.

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>