ARM: at91/dt: sam9261: remove slow RC osc

The at91sam9261 doesn't actually have a slow RC oscillator, remove it from the
dtsi.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>