usb: gadget: nokia: Add mass storage driver to g_nokia

This patch adds removable mass storage support to g_nokia gadget (for N900).
It means that at runtime block device can be exported or unexported.
So it does not export anything by default and thus allows to use MyDocs
partition as before...

[ balbi@ti.com: make it build ]

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>