phy: fsl-imx8-mipi-dphy: Hook into runtime pm

This allows us to shut down the mipi power domain on the SOC. The alternative would be to drop the dphy from the mipi power domain in the SOCs device tree and only have the DSI host controller visible there (and rely on the phy layer's built in runtime pm handling) but this would make the power domain dependency less explicit.

Currently the pm domain can't be shut off when blanking the panel:

pm_genpd_summary
domain                          status          slaves
    /device                                             runtime status
----------------------------------------------------------------------
mipi                            on
    /devices/platform/soc@0/soc@0:bus@30800000/30a00300.dphy  unsupported
    /devices/platform/soc@0/soc@0:bus@30800000/30a00000.mipi_dsi  suspended

while with this change we can shut down the power domain on panel blank:

mipi                            off-0
    /devices/platform/soc@0/soc@0:bus@30800000/30a00300.dphy  suspended
    /devices/platform/soc@0/soc@0:bus@30800000/30a00000.mipi_dsi  suspended

Signed-off-by: Guido Günther agx@sigxcpu.org