soc: arm: replace DT_CPU_CLOCK_FREQUENCY with new dt macros

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>