arm64: dts: renesas: Add Renesas R8A77990 Kconfig support

Add configuration option for the R-Car E3 (R8A77990) SoC.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>