arm64: renesas: Move early SPL stack into System RAM SRAM on R-Car V4H boards

arm64: renesas: Move early SPL stack into System RAM SRAM on R-Car V4H boards

The CONFIG_CUSTOM_SYS_INIT_SP_ADDR=0xeb300000 does not make it into
board .config on either R-Car V4H White Hawk or Sparrow Hawk, remove
this configuration option. The early SPL stack is however pointing
into the RT-VRAM and may corrupt payload loaded into the RT-VRAM by
the BootROM. Set the early SPL stack at fixed location at the end of
System RAM instead, where it cannot interfere with the payload loaded
into RT-VRAM.

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