ARM: shmobile: r8a7791: Add IRQC clock to device tree

Link the external IRQ controller irqc0 to the IRQC module clock, so it
can be power managed using that clock.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>