soc: zte: pm_domains: Add support for zx296718

This patch introduces the power domain driver of zx296718
which belongs to zte's zx2967 family.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>