ARM: shmobile: sh73a0: add irqpin DT nodes

Add DT nodes for the 4 irqpin interrupt controllers on sh73a0. We add them
to sh73a0.dtsi, which is also used by configurations, doing all their
device instantiation from board the .c code. We rely on the fact, that
such configurations don't instantiate devices from the device-tree.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>