Adding support for Orange Pi Zero 3

[electricworry]

I would like to contribute improvements u-boot to enable support for
the Orange Pi Zero 3 board. I have already gained a good understanding
of the project and the standards for changes, and so I have made
modifications and tested them on my board.

Before submitting patches for review I've got a few questions I'd like
to discuss.

The board is an Allwinner H618. From what I can gather this is just an
H616 with minor changes. The changes are so minor that I've been able
to implement the support as changes to the existing files for H616.
For example, to add LPDDR4 support I have been able to make changes to
arch/arm/mach-sunxi/dram_sun50i_h616.c and done so in a well
configured way that won't impact anything else. Is doing it in this
way acceptable or would the preference be to have a separate file
(e.g. dram_sun50i_h618.c) to keep H616 and H618 separate? The way I've
done it feels right because it really is just a minor upgrade, but
there might be other opinions I should hear first.

Further, with regards to attribution, all of my changes are based on
code in https://github.com/orangepi-xunlong/u-boot-orangepi. I've not
been able to cherry-pick changes because that codebase has departed
severely from the u-boot master branch and it's not as elegantly
maintained, so I've had to surgically move just the changes required
to get my board working. Their repo is correctly marked as GPL-2.0 so
compatible with merging, but the exact author of every line of code
will be difficult to attribute. Is it sufficient for me to cite that
downstream repo as the source of the patches?

Thanks in advance. I've not submitted a patch here before so I could
benefit from some hand-holding to not create problems for people, but
I'm pretty confident that the changes I've made comply with the
standards of the project and that there won't be much of an issue
having them accepted.