sh: SH7786 SMP support.

SH7786 is roughly identical to SH-X3 proto SMP, though there are only 2
CPUs. This just wraps in to the existing SH-X3 SMP code with some minor
changes for SH7786, including wiring up the IPIs properly, enabling
IRQ_PER_CPU, and so forth.

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