dm: implement a Timer uclass

Implement a Timer uclass to work with lib/time.c.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/Makefile b/drivers/Makefile
index 1a30ad1..4f49bfd 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -48,6 +48,7 @@
 obj-y += dfu/
 obj-y += rtc/
 obj-y += sound/
+obj-y += timer/
 obj-y += tpm/
 obj-y += twserial/
 obj-y += video/