Invalidate INIT_RAM TLB mappings

Invalidate INIT_RAM TLB mappings

Commit 0db37dc...  (and some others) changed the INIT_RAM TLB
mappings to be unguarded.  This collided with an existing "bug"
where the mappings for the INIT_RAM were being kept around.
This meant that speculative loads to those addresses were
succeeding in the TLB, and going out to the bus, where they
were causing an exception (there's nothing at that address). The
Flash code was coincidentally causing such a speculative load.
Rather than go back to mapping the INIT RAM as guarded, we fix
it so that the entries for the INIT_RAM are invalidated.  Thus
the speculative loads will fail in the TLB, and have no effect.

Signed-off-by: Andy Fleming <afleming@freescale.com>