arm64: renesas: Extend stub PSCI implementation to R-Car Gen5

arm64: renesas: Extend stub PSCI implementation to R-Car Gen5

Extend the stub PSCI implementation with support for R-Car Gen5.
R-Car Gen5 uses different register to perform reset, therefore
add a compile-time conditional. This is __secure code, therefore
the runtime SoC detection parts are not available to it, hence
the compile time conditional.

Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org> # Tweak commit message