dt-bindings: arm: sunxi: add binding for Lichee Zero Plus core board

The Lichee Zero Plus is a core board made by Sipeed, with a microUSB
connector on it, TF slot or WSON8 SD chip, optional eMMC or SPI Flash.
It has a gold finger connector for expansion, and UART is available from
reserved pins w/ 2.54mm pitch. The board can use either SoChip S3 or
Allwinner V3L SoCs.

Add the device tree binding of the basic version of the core board --
w/o eMMC or SPI Flash, w/ TF slot or WSON8 SD, and use S3 SoC.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>