x86: add TSC_CONSTANT_RATE option

Some boards use the local apic for udelay(), but they also provide
their own implementation of udelay() for SMM. The reason for using
the local apic for udelay() in ramstage is to not have to pay the
penalty of calibrating the TSC frequency. Therefore provide a
TSC_CONSTANT_RATE option to indicate that TSC calibration is not
needed. Instead rely on the presence of a tsc_freq_mhz() function
provided by the cpu/board.  Additionally, assume that if
TSC_CONSTANT_RATE is selected the udelay() function in SMM will
be the tsc.

Change-Id: I1629c2fbe3431772b4e80495160584fb6f599e9e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3168
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/cpu/x86/tsc.h

@@ -40,4 +40,8 @@ static inline unsigned long long rdtscll(void)
 }
 #endif
 
+#if CONFIG_TSC_CONSTANT_RATE
+unsigned long tsc_freq_mhz(void);
+#endif
+
 #endif /* CPU_X86_TSC_H */