ARM: at91: move sama5d3 SoC to common clk

This patch removes the selection of AT91_USE_OLD_CLK when selecting sama5d3
SoC support. This will enable automatically enable COMMON_CLK_AT91 option
and add support for at91 common clk implementation.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>