ARM: dts: r9a06g032: Correct the GIC DT node name

Harmless mistake, but it's incorrect. The DT spec provides recommendations
for the node names:
"The name of a node should be somewhat generic, reflecting the function
of the device and not its precise programming model. If appropriate, the
name should be one of the following choices:
...
interrupt-controller"

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>