ARM: dts: r8a7791: Add INTC-SYS clock to device tree

Link the ARM GIC to the INTC-SYS module clock, and add it to the "always
on" PM Domain, so it can be power managed using that clock.

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>