cpufreq: SPEAr: Add CPUFreq driver

SPEAr is an ARM based family of SoCs. This patch adds in support of cpufreq
driver for SPEAr SoCs. It is supported via DT only and so bindings are present
in binding document.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>