sh: extend INTC with force_enable

Extend the shared INTC code with force_enable support to
allow keeping mask bits statically enabled. Needed by
upcoming INTC SDHI patches that mux together a bunch of
vectors to a single linux interrupt which is masked by
a priority register, but needs individual mask bits
constantly enabled.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>