cpufreq: qoriq: Add ls1028a chip support

Enable cpufreq feature on ls1028a chip by adding its compatible
string.

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