fbdev: mx3fb: Remove the driver

The mx3fb driver does not support devicetree and i.MX has been converted
to a DT-only platform since kernel 5.10.

As there is no user for this driver anymore, just remove it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Helge Deller <deller@gmx.de>