x86: Add function to read time stamp counter

Put this function in the u-boot-x86.h header file. We could instead create
timer.h perhaps.

We support setting a base time, and reading the time relative to this base.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>