arm64: dts: renesas: r8a77990: Add CPUIdle support for CA53 cores

Enable cpuidle (core shutdown) support for the CA53 cores on R-Car E3.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200309171200.21226-1-geert+renesas@glider.be