dm: Simple Watchdog uclass

This is a simple uclass for Watchdog Timers. It has four operations:
start, restart, reset, stop. Drivers must implement start, restart and
stop operations, while implementing reset is optional: It's default
implementation expires watchdog timer in one clock tick.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index e956915..b15f1d0 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -42,4 +42,5 @@
 obj-$(CONFIG_DM_VIDEO) += video.o
 obj-$(CONFIG_ADC) += adc.o
 obj-$(CONFIG_SPMI) += spmi.o
+obj-$(CONFIG_WDT) += wdt.o
 endif