watchdog: Allow to use CONFIG_WDT without starting watchdog

In some cases it is useful to compile support for U-Boot command 'wdt'
without starting HW watchdog in early U-Boot phase. For example when the
user want to start the watchdog only on demand by some boot script.

This change adds a new compile option WATCHDOG_AUTOSTART to control whether
U-Boot should automatically start the watchdog during init phase or not.

This option is enabled by default as it was the default behavior prior
introducing this new change. When compiling U-Boot users can decide to turn
this option off.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/drivers/watchdog/wdt-uclass.c b/drivers/watchdog/wdt-uclass.c
index 7500b3e..0603ffb 100644
--- a/drivers/watchdog/wdt-uclass.c
+++ b/drivers/watchdog/wdt-uclass.c
@@ -51,6 +51,11 @@
 						    4 * reset_period) / 4;
 	}
 
+	if (!CONFIG_IS_ENABLED(WATCHDOG_AUTOSTART)) {
+		printf("WDT:   Not starting\n");
+		return 0;
+	}
+
 	ret = wdt_start(gd->watchdog_dev, timeout * 1000, 0);
 	if (ret != 0) {
 		printf("WDT:   Failed to start\n");