dt-bindings: power: gpcv2: add power-domains property

Some pgc power-domain requires a parent power domain, so
add an optional power-domains property, otherwise there will be
dt check warning:
gpc@303a0000: pgc:power-domain@1: 'power-domains' does not match
any of the regexes: 'pinctrl-[0-9]+'

Fixes: 30af8513 ("dt-bindings: power: add defines for i.MX8MM power domains")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>