ARM: shmobile: r8a7740: Add OF support to initialze the GIC

We add a variant to initalize the interrupt controller in case we describe
the GIC using the Device Tree and not platform data.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>