docs: add Orange Pi PC document

docs: add Orange Pi PC document

The Xunlong Orange Pi PC machine is a functional ARM machine
based on the Allwinner H3 System-on-Chip. It supports mainline
Linux, U-Boot, NetBSD and is covered by acceptance tests.

This commit adds a documentation text file with a description
of the machine and instructions for the user.

Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200311221854.30370-19-nieklinnenbank@gmail.com
[PMM: moved file into docs/system/arm to match the reorg
of the arm target part of the docs; tweaked heading to
match other boards]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>