configs: starfive: Enable environment in SPI flash support

On Starfive Visionfive 2, the u-boot environment settings are saved to
on-board SPI flash. Enable relative configs by default and set offset
and size according to upstream linux dts.

Signed-off-by: Shengyu Qu <wiagn233@outlook.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>