video: fsl_dcu_fb: add DM_VIDEO support

Extend the driver to build with DM_VIDEO enabled. DTS files
must additionally include 'u-boot,dm-pre-reloc' property in
soc and child nodes to enable driver binding to fsl_dcu_fb device.

Currently display timings aren't obtained from DT.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>