sh: intc - add single bitmap register support

This patch adds single bitmap register support to intc. The current
code only handles 16 and 32 bit registers where a set bit means
interrupt enabled, but this is easy to extend in the future.

The INTC_IRQ() macro is also added to provide a way to hook in
interrupt controllers for FPGAs in boards or companion chips.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>