sh: Support early IRQ vector map reservation for delayed controllers.

Some controllers will need to be initialized lazily due to pinmux
constraints, while others may simply have no need to be brought online if
there are no backing devices for them attached. In this case it's still
necessary to be able to reserve their hardware vector map before dynamic
IRQs get a hold of them.

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