sh: Wire up INTC subgroup splitting for SH7786 SCIF1.

SH7786 is the big user for subgroup splitting, mostly for the PCIe block,
but those will follow later. For now we simply split up SCIF1, as used by
the serial console on SDK7786 and others.

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