ARM: dts: imx6sx: Describe the default LCDIF1 parent

A suitable default for the LCDIF parent is the PLL5 clock, so
describe it in the device tree.

The imx6sx clock driver harcodes PLL5 as the LCDIF1 parent, but
in preparation for removing such hardcoding, describe the parent
relationship via devicetree.

There are some boards that may want to use a different parent
for the LCDIF due to EMI reasons, for example.

With this approch, the user can change the LCDIF parent in the board
devicetree if needed.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>