cpufreq: Kconfig: Remove architecture specific menu entries

CPUFreq driver's Kconfig entries are added in Kconfig.<arch> files and they are
all included from the main Kconfig file using a menu entry. This creates another
level of (unnecessary) hierarchy within the menuconfig entries.

The problem occurs when there are drivers usable across architectures. Either
their config entry is duplicated in all the supported architectures or is put
into the main Kconfig entry. With the later one, we have menuconfig entries for
drivers at two levels then.

Fix these issues by getting rid of another level of menuconfig hierarchy and
populate all drivers within the main cpufreq menu. To clearly distinguish where
the drivers start from, also add a comment that will appear in menuconfig.

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