ARM: at91: rm9200: fix clock registration

Actually register clocks from device tree when using the common clock
framework.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[nicolas.ferre@atmel.com: add at91 to function name]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>