arm64: dts: imx8mp: Fix power-domain typo

dt_binding_check detects an issue with the pgc_hsiomix power
domain:
  pgc: 'power-domains@17' does not match any of the regexes

This is because 'power-domains' should be 'power-domain'

Fixes: 2ae42e0c ("arm64: dts: imx8mp: add HSIO power-domains")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>