arch/x86: allow timestamp source to not be TSC

Some x86 platforms don't have a TSC that is invariant w.r.t.
rate to get accurate timestamps. As such a different timestamp
is required. Therefore, allow one to specify non-TSC timestamp
source and not compile in the default x86 TSC code.

BUG=b:72378235,b:72170796

Change-Id: I737fcbba60665b3bc2b5864269536fda78b44d90
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/23423
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

src/arch/x86/Kconfig

@@ -271,3 +271,18 @@ config ACPI_CPU_STRING
 	  Sets the ACPI name string in the processor scope as written by
 	  the acpigen function. Default is \_PR.CPxx. Note that you need
 	  the \ escape character in the string.
+
+config COLLECT_TIMESTAMPS_NO_TSC
+	bool
+	default n
+	depends on COLLECT_TIMESTAMPS
+	help
+	  Use a non-TSC platform-dependent source for timestamps.
+
+config COLLECT_TIMESTAMPS_TSC
+	bool
+	default y if !COLLECT_TIMESTAMPS_NO_TSC
+	default n
+	depends on COLLECT_TIMESTAMPS
+	help
+	  Use the TSC as the timestamp source.