soc: imx: gpc: add workaround for i.MX6QP to the GPC PD driver

On i.MX6QP, due to hardware erratum ERR009619, the PRE clocks may be
stalled during the power up sequencing of the PU power domain. As this
may lead to a complete loss of display output, the recommended
workaround is to keep the PU domain enabled during normal system
operation.

Implement this by rejecting the domain power down request on the
affected SoC.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>