zephyr: #ifdef code in `common/timer.c` not used in Zephyr
zephyr: #ifdef code in `common/timer.c` not used in Zephyr

Several functions in `common/timer.c` are not used in Zephyr-based
builds. Exclude these with `#ifndef CONFIG_ZEPHYR`.

BRANCH=None
BUG=b:247608485
TEST=./twister
LOW_COVERAGE_REASON=No new code; Minor re-org of non-Zephyr functions

Signed-off-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>
Change-Id: I2858f8b9b59b279f8b786abca26a3b2471db59cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3910979
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>

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