ARM: at91: move at91sam9260 SoCs to the CCF

This patch removes the selection of AT91_USE_OLD_CLK when selecting at91sam9260
SoCs support. This will automatically enable COMMON_CLK_AT91 option and add
support for at91 common clk implementation.

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