MAINTAINERS: Add maintainer for hisilicon DRM driver

Add maintainer and reviewer for hisilicon DRM driver.

v8:
- Append "/" to directory patterns.
v7: None.
v6: None.
v5: None.
v4:
- Add Chen Feng <puck.chen@hisilicon.com> as Designated reviewer.
v3: First version.

Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>