ums: allow the user to specify the device type

Allow an optional devtype parameter to the ums command, which specifies
the type of the device to be exported. This could allow exporting a SATA
or even another USB device.

Cc: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
WIP: ARM: tegra: USB device mode/mass storage board enablement

Signed-off-by: Stephen Warren <swarren@nvidia.com>