pmdomain: imx93-blk-ctrl: add support for optional subnodes

pmdomain: imx93-blk-ctrl: add support for optional subnodes

This particular block can have DT subnodes describing the LVDS LDB, MIPI
DSI and parallel DPI bridge.

Scan for possible sub-devices within the driver, instead of misusing the
simple-bus to perform the scan.

Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>