ARM: shmobile: Use r8a7790 suffix for IRQC compat string

Add "renesas,irqc-r8a7790" to the compatible string for IRQC
in case of r8a7790. This makes the IRQC 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>