[ppc4xx] Extend program_tlb() with virtual & physical addresses

Now program_tlb() allows to program a TLB (or multiple) with
different virtual and physical addresses. With this change, now one
physical region (e.g. SDRAM) can be mapped 2 times, once with caches
diabled and once with caches enabled.

Signed-off-by: Stefan Roese <sr@denx.de>