tegra132: fill out udelay() implementation

There was an empty udelay() implementation result in 0 waits.
Provide an actual implementation.

BUG=None
BRANCH=None
TEST=Built and ran through to depthcharge on rush.

Change-Id: I201f2fdc4e4f5c88d48e4002839b03e808a5a1bc
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/210827
Reviewed-by: Furquan Shaikh <furquan@chromium.org>

src/soc/nvidia/tegra132/timer.c

@@ -28,4 +28,18 @@ void init_timer(void)
 
 void udelay(unsigned usec)
 {
+	struct mono_time current, end;
+
+	if (!thread_yield_microseconds(usec))
+		return;
+
+	/*
+	 * As the hardware clock granularity is in microseconds pad the
+	 * requested delay by one to get at least >= requested usec delay.
+	 */
+	timer_monotonic_get(&end);
+	mono_time_add_usecs(&end, usec + 1);
+	do {
+		timer_monotonic_get(&current);
+	} while (mono_time_before(&current, &end));
 }