x86: Add TSC timer

This timer runs at a rate that can be calculated, well over 100MHz. It is
ideal for accurate timing and does not need interrupt servicing.

Tidy up some old broken and unneeded implementations at the same time.

To provide a consistent view of boot time, we use the same time
base as coreboot. Use the base timestamp supplied by coreboot
as U-Boot's base time.

Signed-off-by: Simon Glass <sjg@chromium.org&gt;base>
Signed-off-by: Simon Glass <sjg@chromium.org>