sunxi: defaultly enable SPL for Lichee Pi Zero

As we have already DRAM initialization code for V3s SoC, we can
defaultly enable SPL now on Lichee Pi Zero.

Add CONFIG_SPL in Lichee Pi Zero defconfig.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jagan Teki <jagan@openedev.com>