sh: Initial SH-X3 SMP support.

This adds basic support for SH-X3 SMP (4 CPUs).

More IPI and cache debugging is necessary, mostly interfacing the
d-cache coherency and the I-cache broadcast invalidates. Only for
testing at present!

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