x86/tsc: Add X86_FEATURE_TSC_KNOWN_FREQ flag

The X86_FEATURE_TSC_RELIABLE flag in Linux kernel implies both reliable
(at runtime) and trustable (at calibration). But reliable running and
trustable calibration independent of each other. 

Add a new flag X86_FEATURE_TSC_KNOWN_FREQ, which denotes that the frequency
is known (via MSR/CPUID). This flag is only meant to skip the long term
calibration on systems which have a known frequency.

Add X86_FEATURE_TSC_KNOWN_FREQ to the skip the delayed calibration and
leave X86_FEATURE_TSC_RELIABLE in place.

After converting the existing users of X86_FEATURE_TSC_RELIABLE to use
either both flags or just X86_FEATURE_TSC_KNOWN_FREQ we can seperate the
functionality.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Bin Gao <bin.gao@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1479241644-234277-2-git-send-email-bin.gao@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>