ARM: dts: imx6: RDU2: assign video PLL as input to LDB

Currently we don't ensure that the LDB is clocked from the video PLL
and relied on the bootloader to do the correct setup. This isn't always
true, in which case we would run with a vastly different video clock
than the desired one. Fix this by assigning the proper parent to the
LDB.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>