rockchip: rk3566-anbernic-rgxx3: Remove SPL_ROCKCHIP_BACK_TO_BROM option

rockchip: rk3566-anbernic-rgxx3: Remove SPL_ROCKCHIP_BACK_TO_BROM option

SPL_ROCKCHIP_BACK_TO_BROM should normally only be enabled when BROM
should load U-Boot binary. SPL on Anbernic RGxx3 devices load TF-A and
U-Boot proper from FIT images and does never jump back to BROM from SPL.

Remove the superfluous Kconfig option from defconfig to align with other
RK356x boards.

This patch have no intended change in boot behavior.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>