host: add __hw_clock_source_read
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 <1784cbba6d7675b1eba55eaefe41c7672ad9da3b@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1055191
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec