clk: qcom: clk-debug: Add custom measure operators for mccc_clk

Since the generic way of using testclock to measure ddr mc clock
is not available, rely on reading the period of the current mc clock
frequency written to by DDRSS software. This does not provide the
physical rate at which the clock is running, but a snapshot of the
current configuration.

Change-Id: I37ce2927be7712adc2b722ed0a26c8eaf4596dd2
Signed-off-by: David Dai <daidavid1@codeaurora.org>