clk: ux500: Add device tree bindings for ABx500 clocks

This adds device tree bindings for the ABx500 clocks on the
ST-Ericsson platforms.

Cc: devicetree@vger.kernel.org
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>