ARM: shmobile: Use r8a7778 suffix for INTC compat string

Add "renesas,intc-irqpin-r8a7778" to the compatible string for the
IRQ pins in case of r8a7778 INTC. This makes the INTC irqpin follow
the same style as the other devices and also makes it more future proof.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>