MAINTAINERS: update ARM/ZTE entry

Update ARM/ZTE entry to include all those ZTE ZX platform drivers and
files that do not have a separate MAINTAINERS entry.

Since I have been contributing a lot for ZTE ZX SoCs support and
collecting platform patches for a few cycles, add myself as one
maintainer for the platform.

Cc: Jun Nie <jun.nie@linaro.org>
Cc: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>