ARM: u300: convert to common clock

This converts the U300 clock implementation over to use the common
struct clk and moves the implementation down into drivers/clk.
Since VCO isn't used in tree it was removed, it's not hard to
put it back in if need be.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[mturquette@linaro.org: trivial Makefile conflict]
Signed-off-by: Mike Turquette <mturquette@linaro.org>