ARM: dts: add the AB8500 sysclk to the device trees

This clock has been missing since some early stages of device tree
conversion. Adding the right clocks to the device tree makes USB
work again.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>