ARM: mx28: set proper parent for lcdif clock

Most likely, the LCD panel on mx28 platform will require a pixel
clock higher than ref_xtal_clk (24 MHz), so the patch initializes
the parent of lcdif clock as ref_pix_clk.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>