dt-bindings: soc: imx8mp-media-blk-ctrl: Add LDB subnode into schema and example

Document the LDB bridge subnode and add the subnode into the example.
For the subnode to work, the block control must scan its subnodes and
bind drivers to them, do not misuse either simple-bus or simple-mfd
here.

Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Liu Ying <victor.liu@nxp.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>