tegra124: Implement the monotonic timer by reading the 1us timer register.

It turns out there's a register in tegra which automatically counts at 1us
increments. It's primarily intended for hardware to use (I think to drive
other timers) but we can read it ourselves since a 1us timer is exactly what
we need to support the monotonic timer API.

BUG=None
TEST=Built and booted into the bootblock on nyan with test code in the
bootblock. Used that code to print two serial messages, 1 minute apart. The
messages happened 1 minute and 2 seconds apart. That's pretty close, although
it might be worthwhile to figure out where those extra two seconds are coming
from (measurement error?).
BRANCH=None

Change-Id: I68e947944acec7b460e61f42dbb325643a9739e8
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/172044
Reviewed-by: Ronald Minnich <rminnich@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>

src/soc/nvidia/tegra124/monotonic_timer.c

@@ -17,8 +17,11 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <arch/io.h>
+#include <soc/addressmap.h>
 #include <timer.h>
 
 void timer_monotonic_get(struct mono_time *mt)
 {
+	mono_time_set_usecs(mt, read32((void *)TEGRA_TMRUS_BASE));
 }