firmware: imx: scu-pd: Update comments for single global power domain

Since the introduction of the PM domain support for the scu-pd, the genpd
framework has been continuously improved. More preciously, using a single
global power domain can quite easily be deployed for imx platforms.

To avoid confusions, let's therefore make an update to the comments about
the missing pieces.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>