phycore-imx8mp: add support for booting and flashing emmc via UUU

add support for Serial Downloader Boot via UUU as well as flashing emmc
via UUU on USB0 Port of phyBOARD Pollux.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>