clk: at91: add PMC smd clock

This patch adds at91 smd (Soft Modem) clock implementation using common clk
framework.

Not used by any driver right now.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>