sh: Split out MMUCR.URB based entry wiring in to shared helper.

Presently this is duplicated between tlb-sh4 and tlb-pteaex. Split the
helpers out in to a generic tlb-urb that can be used by any parts
equipped with MMUCR.URB.

At the same time, move the SH-5 code out-of-line, as we require single
global state for DTLB entry wiring.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>