powerpc/mpc85xx: Add clear_ddr_tlbs function

This is useful when we just want to wipe out the TLBs.  There's currently
a function that resets the ddr tlbs to a different value; it is changed to
utilize this function.  The new function can be used in conjunction with
setup_ddr_tlbs() for a board to temporarily map/unmap the DDR address
range as needed.

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