ARM: dts: r8a7779: Add device node for ARM global timer

Add a device node for the global timer, which is part of the Cortex-A9
MPCore.

The global timer can serve as an accurate (4 ns) clock source for
scheduling and delay loops.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191211135222.26770-4-geert+renesas@glider.be