soc: snps_arc_iot: replace DT_SNPS_ARCEM_0_CLOCK_FREQUENCY w/new macros

Replace DT_SNPS_ARCEM_0_CLOCK_FREQUENCY with a PATH based reference
to cpu@0 (DT_PATH(cpus, cpu_0)) and than getting the clock_frequency
property:

    DT_SNPS_ARCEM_0_CLOCK_FREQUENCY ->
        DT_PROP(DT_PATH(cpus, cpu_0), clock_frequency)

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>