rtc: at91sam9: use clk API instead of relying on AT91_SLOW_CLOCK

The RTT block is using the slow clock which is accessible through the clk
API.
Use the clk API to retrieve, enable and get the slow clk rate instead of
the AT91_SLOW_CLOCK macro (which hardcodes the slow clk rate).
Doing this allows us to reference the clk thus preventing the CCF from
disabling it during the "disable unused" phase.

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>