sh: __irq_entry annotate do_IRQ().

This adds an __irq_entry annotation for do_IRQ() so that the IRQ
annotation in the function graph tracer works as advertized. We already
have the IRQENTRY section wired up, so this is just a trivial addition
to actually make use of it.

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