Add RTT bindings documentation.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: NJohan Hovold <johan@kernel.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>