ARM: renesas: Set CONFIG_SYS_TEXT_BASE=0x0 on R-Car Gen3

Since R-Car Gen3 already enables position independent build, also set
CONFIG_SYS_TEXT_BASE=0x0 to finalize the switch. This is possible since
534f0fbd ("arm64: Fix relocation of env_addr if POSITION_INDEPENDENT=y")
fixed current env_get_char() crash with CONFIG_SYS_TEXT_BASE=0x0 .

This change permits us to start U-Boot from any location in DRAM instead
of specific TEXT_BASE.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>