ARM: dts: imx6ul: Add clock and PGC node to GPC

According to fsl,imx-gpc.yaml the General Power Control requires
a ipg clock and a Power Gating Control node. So add them to fix
the dtbs_check warnings on i.MX6UL boards:

gpc@20dc000: 'clocks' is a required property
gpc@20dc000: 'clock-names' is a required property
gpc@20dc000: 'pgc' is a required property

Suggested-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>