ARM: at91: add new compatible strings for pmc driver

This patch adds new compatible string for PMC node to prepare the
transition to common clk.

These compatible string come from pmc driver in clk subsystem and are
needed to provide new device tree compatibility with old at91 clks
(device tree using common clks will use the new compatible strings).

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>