cpufreq: qoriq: Don't look at clock implementation details

Get the CPU clock's potential parent clocks from the clock interface
itself, rather than manually parsing the clocks property to find a
phandle, looking at the clock-names property of that, and assuming that
those are valid parent clocks for the cpu clock.

This is necessary now that the clocks are generated based on the clock
driver's knowledge of the chip rather than a fragile device-tree
description of the mux options.

We can now rely on the clock driver to ensure that the mux only exposes
options that are valid.  The cpufreq driver was currently being overly
conservative in some cases -- for example, the "min_cpufreq =
get_bus_freq()" restriction only applies to chips with erratum
A-004510, and whether the freq_mask used on p5020 is needed depends on
the actual frequencies of the PLLs (FWIW, p5040 has a similar
limitation but its .freq_mask was zero) -- and the frequency mask
mechanism made assumptions about particular parent clock indices that
are no longer valid.

Signed-off-by: Scott Wood <scottwood@nxp.com>
Signed-off-by: Tang Yuantian <yuantian.tang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>