* Update the RockChip sub-maintainer from rkchrome to rockchip-linux
in maintainers.rst.

* Add missing documentation files and change extensions from `md` to `rst`.

* Add sub-maintainer for Socionext UniPhier platform.

Change-Id: I7f498316acb0f7947c6432dbe14988e61a8903fe
Co-Authored-By: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Dan Handley <dan.handley@arm.com>