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Oscillators, PLLs and some system clocks can be related straight to
a parent clock. Prior this change were only oscillators and few
clocks supported by this look up. This changes adds PLLs and other
system clocks. This enables for flexible use of clock tree exploration
when computing a clock frequency value.

Change-Id: I15ec98023a7095e3120a6954de59a4799d92c66b
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>