soc: imx: gpcv2: split power up and power down sequence control

The current mixed function to control both power up and power down
sequences is very hard to follow and already contains some sequence
errors like triggering the ADB400 handshake at the wrong time due to
this. Split the function into two, which results in slightly more
code, but is way easier to get right.

Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>