MAINTAINERS: add zx2967 SoC drivers to ARM ZTE architecture

Add the ZTE SoC drivers as maintained by ARM ZTE
architecture maintainers, as they're parts of the core IP.

By the way, this patch adds the maintainer for ARM
ZTE architecture to Baoyou Xie.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>