cpufreq: qoriq: enhance bus frequency calculation

On some platforms, property device-type may be missed in soc node
in dts which caused the bus-frequency can not be obtained correctly.

This patch enhanced the bus-frequency calculation. When property
device-type is missed in dts, bus-frequency will be obtained by
looking up clock table to get platform clock and hence get its
frequency.

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