soc: imx6qp: Use GENPD_FLAG_ALWAYS_ON for PU errata

This is functionally identical but simpler and slightly faster.

The PU domain is turned on at boot time and never turned off. In the
current implementation the pm core will repeatedly call power_off when
the domain is unused and get -EBUSY back. If the domain is marked as
"always on" instead the pm core won't even attempt to turn it off.

In theory on 6qp it is safe to turn PU off in suspend, however that is
best accomplished with a new core flag.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>