cpufreq: introduce a hardlimit that will be respected no matter what …

…userspace is trying to force (Yank555.lu)

SysFS interface :

 * /sys/kernel/cpufreq/hardlimit (rw)
 *
 *   set or show the real hard CPU frequency limit
 *
 * /sys/kernel/cpufreq/version (ro)
 *
 *   display CPU freq hard limit version information

Conflicts:
	drivers/cpufreq/Kconfig
	drivers/cpufreq/cpufreq.c