sh: Bring SMP support back from the dead.

There was a very preliminary bunch of SMP code scattered around for the
SH7604 microcontrollers from way back when, and it has mostly suffered
bitrot since then. With the tree already having been slowly getting
prepped for SMP, this plugs in most of the remaining platform-independent
bits.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>