sh: Guard against early IPIs in flush_cache_all().

flush_cache_all() gets called in to when we do some early ioremapping.
Unfortunately on SDK7786 the interrupt controller itself requires
ioremapping, leading to a bit of a chicken and egg scenario. For now,
don't bother with IPI crosscalls if there aren't any other CPUs online.

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