MAINTAINERS: git-mailrc: update maintainer for Rockchip

Send patch to Kever Yang instead of Lin Huang for Rockchip patches,
for Lin is not always working on upstream U-Boot.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Changed , to : in subject:
Signed-off-by: Simon Glass <sjg@chromium.org>