USB: serial: add Google simple serial SubClass support

Add support for Google devices that export simple serial
interfaces using the vendor specific SubClass/Protocol pair
0x50/0x01.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
[johan: move id entries and update Kconfig]
Signed-off-by: Johan Hovold <johan@kernel.org>