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>
diff --git a/arch/arm/cpu/armv7/omap-common/timer.c b/arch/arm/cpu/armv7/omap-common/timer.c
index 9f8bc93..dcc40bb 100644
--- a/arch/arm/cpu/armv7/omap-common/timer.c
+++ b/arch/arm/cpu/armv7/omap-common/timer.c
@@ -118,3 +118,8 @@
 {
 	return CONFIG_SYS_HZ;
 }
+
+unsigned long timer_get_us(void)
+{
+	return get_ticks();
+}