soc: mediatek: SCPSYS: Fix double enabling of regulators

With CONFIG_PM enabled do not call genpd->power_on manually as this
will cause the regulators being turned on once in SCPSYS probe and
then again when the genpd core turns on the domains. Instead, call
genpd->power_on only with CONFIG_PM disabled and tell the genpd core
that the domains are disabled when registered.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>