ppc/85xx: Add a simple function to search the TLB

Allow us to search the TLB array based on an address.  This is useful
if we want to change an entry but dont know where it happens to be
located.

For example, the boot page mapping we use on MP or the flash TLB that
we change the WIMGE settings for after we've relocated.

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