ARM: shmobile: r8a7791 dtsi: Use "arm,gic-400" for GIC

Replace the "arm,cortex-a15-gic" compatible value for the GIC by
"arm,gic-400", as the R-Car Gen2 GIC is assumed to be a GIC-400.
This has been confirmed by reading the GICD_IIDR register, which reports
0x0200043b (GIC-400 = 0x02, ARM = 0x43b).

This has no effect on runtime behavior, as currently the GIC driver
treats both compatible values the same.

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