ARM: at91: move sam9n12 SoC to the CCF

This patch removes the selection of AT91_USE_OLD_CLK when selecting
sam9n12 SoC support. This will automatically enable COMMON_CLK_AT91 option
and add support for at91 common clk implementation.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>