hwtimer: Tidy up and clarify some hw_clock comments
hwtimer: Tidy up and clarify some hw_clock comments

From what I can tell the counter has to tick over at the rate of 1MHz.
Update the comments to make that clear.

BUG=chromium:876737
BRANCH=none
TEST= make buildall -j50

Change-Id: Ib04731c10a68c544973b810cf70ce9ffba556b89
Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1185230

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