arm64: renesas: Make stub PSCI implementation available on 64bit R-Car SoCs

arm64: renesas: Make stub PSCI implementation available on 64bit R-Car SoCs

Make the R-Car V3U stub PSCI implementation available on 64bit R-Car SoCs.
This implementation is useful during early board bring up, where it can
supplant missing fully-featured PSCI implementation. Note that this PSCI
implementation is very basic and offers only SoC reset functionality. It
is unable to enable or disable secondary CPU cores nor does it offer any
suspend/resume functionality.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>