irqdomain: Export remaining public API symbols.

modules making use of irq domains at the very least need access to the
add/remove/lookup routines, though there's nothing preventing them from
using the remainder of the public API, either.

The current set of exports seem primarily geared at DT-enabled platforms
using DT-backed IRQ domains, where many of the API accesses are hidden
away in OF code. The non-DT cases need to do most of this on their own.

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