arm64: dts: renesas: r8a779g0: Add CPUIdle support

Support CPUIdle for ARM Cortex-A76 on R-Car V4H.

Based on patches in the BSP by Tho Vu and Vincent Bryce.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f6d4076983eb45cf23595a045747f28cbdcdf4e6.1668429870.git.geert+renesas@glider.be