x86: tsc: Introduce config option for early timer frequency

So far the TSC timer driver supports trying hardware calibration first
and using device tree as last resort for its running frequency as the
normal timer.

However when it is used as the early timer, it only supports hardware
calibration and if it fails, the driver just panics. This introduces
a new config option to specify the early timer frequency in MHz and
it should be equal to the value described in the device tree.

Without this patch, the travis-ci testing on QEMU x86_64 target fails
each time after it finishes the 'bootefi selftest' as the test.py see
an error was emitted on the console like this:

  TSC frequency is ZERO
  resetting ...
  ### ERROR ### Please RESET the board ###

It's strange that this error is consistently seen on the travis-ci
machine, but only occasionally seen on my local machine (maybe 1 out
of 10). Since QEMU x86_64 target enables BOOTSTAGE support which uses
early timer, with this fix it should work without any failure.

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