arm64: renesas: Add R-Car S4 Starter Kit support

arm64: renesas: Add R-Car S4 Starter Kit support

Add support for the R-Car S4 Starter Kit with R8A779F4 SoC support.
This implementation natively uses OF_UPSTREAM to pull in most recent
DT. The defconfig is derived from S4 Spider, with reduced UART baud
rate to 921600 Bdps. The DT alias to rswitch is removed as the alias
should point to rswitch ports, not to rswitch itself, see [1].

[1] https://lore.kernel.org/linux-arm-kernel/20250118111344.361617-5-marek.vasut+renesas@mailbox.org/

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