ARM: dts: Declare clocks as fixed on bcm11351

Declare clocks that are enabled and configured by bootloaders as fixed
rate clocks in the DTS such that device drivers may use standard clock
function calls.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>