83xx/fdt_support: let user specifiy FSL USB Dual-Role controller role

Linux understands "host" (default), "peripheral" and "otg" (broken).
Though, U-Boot doesn't restrict dr_mode variable to these values (think
of renames in future).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>