sh: Remap physical memory into P1 and P2 in pmb_init()

Eventually we'll have complete control over what physical memory gets
mapped where and we can probably do other interesting things. For now
though, when the MMU is in 32-bit mode, we map physical memory into the
P1 and P2 virtual address ranges with the same semantics as they have in
29-bit mode.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>