ARM: dts: r8a73a4: Add INTC-SYS clock to device tree

Link the ARM GIC to the INTC-SYS module clock and the C4 power domain,
so it can be power managed using that clock in the future.

Note that currently the GIC-400 driver doesn't support module clocks nor
Runtime PM, so this must be handled as a critical clock.

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