ARM: dts: imx6sx: add ARM power domain support

Add ARM power domain in PGC.

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>