docs(maintainers): add Julius Werner as Rockchip platform code owner

The two existing plat/rockchip code owners seem to be no longer active
in the project and are not responding to reviews. There have been a
couple of small fixup patches[1][2][3] pending for months that couldn't
be checked in for lack of Code-Owner-Review+1 flag. Add myself to the
code owner list to unblock this bottleneck (I have been deeply involved
in the rk3399 port, at least, so I know most of the code reasonably
well).

[1] https://review.trustedfirmware.org/9616
[2] https://review.trustedfirmware.org/9990
[2] https://review.trustedfirmware.org/10415

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Ic7b2bb73c35a9bea91ff46ee445a22819d2045d9

docs/about/maintainers.rst

@@ -594,6 +594,8 @@ RockChip platform port
 :|G|: `rockchip-linux`_
 :|M|: Heiko Stuebner <heiko@sntech.de>
 :|G|: `mmind`_
+:|M|: Julius Werner <jwerner@chromium.org>
+:|G|: `jwerner-chromium`_
 :|F|: plat/rockchip/
 
 STM32MP1 platform port