common: Move interrupt functions into a new header

These functions do not use driver model but are fairly widely used in
U-Boot. But it is not clear that they will use driver model anytime soon,
so we don't want to label them as 'legacy'.

Move them to a new irq_func.h header file. Avoid the name 'irq.h' since it
is widely used in U-Boot already.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>