Adding support for Orange Pi Zero 2W

[Federico Borgo]

I would like to contribute improvements to u-boot to enable support for the
Orange Pi Zero 2W board.
I think it is not very different from the Orange Pi Zero 3.
The board has an Allwinner H618 SoC.
I already tried to run u-boot on my Zero 2W using a similar configuration
of the Zero 3 with the devicetree file taken from the linux kernel (
sun50i-h618-orangepi-zero2w.dts).

https://github.com/torvalds/linux/blob/master/arch/arm64/boot/dts/allwinner/sun50i-h618-orangepi-zero2w.dts

Thanks in advance.

Federico