cpufreq: SPEAr: Instantiate as platform_driver

As multiplatform build is being adopted by more and more
ARM platforms, initcall function should be used very carefully.
For example, when SPEAr cpufreq driver is enabled on a kernel
booted on a non-SPEAr board, we will get following boot time error:

	spear_cpufreq: Invalid cpufreq_tbl

To eliminate this undesired the effect, the patch changes SPEAr
driver to have it instantiated as a platform_driver. Then it will
only run on platforms that create the platform_device "spear-cpufreq".

This patch also creates platform node for SPEAr13xx boards.

Reported-by: Josh Cartwright <joshc@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>