ARM: shmobile: r8a7778 dtsi: Use "arm,pl390" for GIC

Replace the "arm,cortex-a9-gic" compatible value for the GIC by
"arm,pl390", as the documentation states it is a PL390.
This has been confirmed by reading the GICD_IIDR register, which reports
0x0000043b (PL390 = 0x00, 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>