musb: sunxi: Add support for platform_set_mode

This allows run-time dr_mode switching support via the "mode" musb
sysfs attribute.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>