ARM: dts: imx28: reparent gpmi clock to ref_gpmi

Since ref_gpmi is sourced from pll0 (480MHz), It allows the GPMI
controller to manage High-Speed NAND Timing (edo mode 3,4 and 5).

Co-developed-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>