sh: intc - improve group support

This patch improves intc group support, ie it makes it possible to
group interrupts together and mask / unmask the entire group. This
also works with priorities, so setting a priority for an entire group
is also possible. This patch is needed to properly support certain
processors such as the 7780.

Fixes for NULL pointers in DECLARE_INTC_DESC() are also included.

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