x86: tsc: Add Airmont reference clock values

Per the Intel 64 and IA-32 Architecture Software Developer's Manual,
add the reference clock for Intel Atom Processors based on the Airmont
Microarchitecture (Braswell).

This keeps in sync with Linux kernel commit:
  6fcb41c: x86/tsc_msr: Add Airmont reference clock values

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>