ARM: at91: add clk_lookup entry for RTT devices

ARM: at91: add clk_lookup entry for RTT devices

First export the clk32k clk.
Then add clk_lookup entries for RTT devices so that rtc-at91sam9 driver
can retrieve and manipulate the slow clk.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>