soc: imx: gpc: ARM power domain should be always-on

ARM power domain does NOT support runtime off, always-on
flag should be set to avoid incorrect power state in
pm_genpd_summary:

Before:

root@imx6qpdlsolox:~# cat /sys/kernel/debug/pm_genpd/pm_genpd_summary
domain                          status          slaves
    /device                                             runtime status
----------------------------------------------------------------------
ARM                             off-0

After:

root@imx6qpdlsolox:~# cat /sys/kernel/debug/pm_genpd/pm_genpd_summary
domain                          status          slaves
    /device                                             runtime status
----------------------------------------------------------------------
ARM                             on

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>