clk: ux500: move AB8500 sysclk over to PRCMU clk driver

The AB8500 sysclk is just another PRCMU-controlled clock, there
is no reason why it should be in the ABx500-controlled part of
the clock implementation. Doing this and the corresponding device
tree changes makes USB work on the Ux500 again.

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