soc: imx: imx8m-blk-ctrl: Scan subnodes and bind drivers to them

This particular block can have DT subnodes describing the LVDS LDB
bridge. Instead of misusing simple-bus to scan for those nodes, do
the scan within the driver.

Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>