soc: zte: pm_domains: Prepare for supporting ARMv8 zx2967 family

The ARMv8 zx2967 family (296718, 296716 etc) uses different value
for controlling the power domain on/off registers, Choose the
value depending on the compatible.

Multiple domains are prepared for the family, this patch prepares
the common functions.

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