Convert CONFIG_FSL_DCU_FB to Kconfig

Rename CONFIG_FSL_DCU_FB to CONFIG_VIDEO_FSL_DCU_FB
and convert it to Kconfig.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Reviewed-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Alison Wang <alison.wang@nxp.com>