ARM: at91: prepare common clk transition for sam9n12 SoC

This patch encloses sam9n12 old clk registration in
"#if defined(CONFIG_OLD_CLK_AT91) #endif" sections.

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>