ARM: dts: imx6*-hummingboard2: remove LDB node

It's disabled by default and the data mapping is supposed to be
retrieved from the attached panel driver in mainline.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>