This patch implements write support to usb device with raw partition.
It will be useful for filesystem write support to usb device from
u-boot in future.

Tested with writing kernel image to raw usb disk & booting with usb
read command into ram.

[Note:  run usb part to get info about start sector & number of
sectors on a partition for usb write operation.]

Signed-off-by: Mahavir Jain <mjain@marvell.com>