irqdomain: Make irq_domain_simple_map() static.

Presently irq_domain_simple_map() isn't labelled as static, but there's
no definition for it in the public irqdomain header either. At present
all in-tree ->map users have meaningful work to do, and all others are
using irq_domain_simple_ops directly. Make it static for now, as it can
always be exported and added to the public API later.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>