clk: add bindings for the Ux500 clocks

These Ux500 clocks have been around for years and were never
properly documented. Add the proper binding documentation.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>