DA8xx: MUSB platform device registration

Add the function to register the MUSB platform device.
Additional compile warning fixes by Sekhar Nori.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>