ARM: at91: move sam9261 SoC to common clk

This patch removes the selection of AT91_USE_OLD_CLK when selecting
sam9261 SoCs support. This will automatically enable COMMON_CLK_AT91 option
and add support for at91 common clk implementation.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>