Replace DT_CPU_CLOCK_FREQUENCY with a PATH based reference to cpu@0
(DT_PATH(cpus, cpu_0)) and than getting the clock_frequency property:
DT_CPU_CLOCK_FREQUENCY -> DT_PROP(DT_PATH(cpus, cpu_0), clock_frequency)
This lets us remove DT_CPU_CLOCK_FREQUENCY from dts_fixup.h
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>