host: add __hw_clock_source_read

Sometimes common code needs __hw_clock_source_read, add it.

The implementation is similar of what common/timer.c does to create a ts
for get_time(), but in reverse.

TEST=Unit tests pass again for the next CL
BRANCH=master
BUG=None

Change-Id: I10564abedabe88e4789723bc97bac170ae020c69
Signed-off-by: Alexandru M Stan <amstan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1055191
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

core/host/timer.c

@@ -67,6 +67,11 @@ timestamp_t get_time(void)
 	return ret;
 }
 
+uint32_t __hw_clock_source_read(void)
+{
+	return get_time().le.lo;
+}
+
 void force_time(timestamp_t ts)
 {
 	timestamp_t now = _get_time();