usb: musb: core: added helper function for parsing DT

This adds the function musb_get_mode() to get the DT property "dr_mode"

Signed-off-by: Petr Kulhavy <petr@barix.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Tested-by: David Lechner <david@lechnology.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>