gen: omap: Add timer_get_us to OMAP boards BUG=none TEST=Build u-boot for Beaglebone, compiles/boots BRANCH=none Change-Id: I6d24bdf0b1d002f2ccae2fbf68ae36a166a50dc1 Signed-off-by: Liam McLoughlin <lmcloughlin@chromium.org> Reviewed-on: https://gerrit.chromium.org/gerrit/56497 Reviewed-by: Bill Richardson <wfrichar@chromium.org>