ARM: at91: prepare common clk transition for sam9261 SoC

This patch encloses sam9261 old clk registration in
"#if defined(CONFIG_OLD_CLK_AT91) #endif" sections.

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>