soc: imx: gpcv2: Synchronously suspend MIX domains

In case the following power domain sequence happens, iMX8M Mini always hangs:
  gpumix:on -> gpu:on -> gpu:off -> gpu:on
This is likely due to another quirk of the GPC block. This situation can be
prevented by always synchronously powering off both the domain and MIX domain.
Make it so. This turns the aforementioned sequence into:
  gpumix:on -> gpu:on -> gpu:off -> gpumix:off -> gpumix:on -> gpu:on

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>