soc: imx: gpc: fix imx6sl gpc power domain regression

Commit 721cabf6 ("soc: imx: move PGC handling to a new GPC driver")
broke the MX6SL GPC power domain support.
It always got the following error:
[    1.248364] imx-gpc 20dc000.gpc: could not find pgc DT node
This patch adds back the legecy support.

Fixes: 721cabf6 ("soc: imx: move PGC handling to a new GPC driver")
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>