ARM: imx6: remove unit address from LDB node

The LDB has no reg property as it uses the IOMUX-GPR syscon to access
its registers. Remove the unit address from the DT node to make DT
compiler happy.

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