soc/amd/stoneyridge: Convert monotonic timer

Use the TSC for the Stoney Ridge monotonic timer.  Modern AMD
CPUs have invariant timestamp counters.  This patch brings the
feature more in line with other devices and allows the use of
typical monotonic timer functions.

BUG=chrome-os-partner:62578062

Change-Id: I07b05fbc7cdea54a45daac01954284a9fd67e42f
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/20201
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
3 files changed