watchdog: introduce separate SPL symbol for WDT_GPIO

Currently, enabling WDT_GPIO on a board which uses SPL, but does not
have SPL_WDT, SPL_DM_GPIO or SPL_OF_CONTROL enabled, breaks the build.

Make it possible to use the WDT_GPIO driver on such boards by
introducing a separate symbol controlling whether the driver is built
for SPL. Make it default to WDT_GPIO such that boards that already
have it enabled and all the SPL prerequisites satisfied will continue
to have it in SPL.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index b6f7e4e..0e45f0a 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -191,6 +191,15 @@
 	  doc/device-tree-bindings/watchdog/gpio-wdt.txt for
 	  information on how to describe the watchdog in device tree.
 
+config SPL_WDT_GPIO
+	bool "External gpio watchdog support in SPL"
+	depends on SPL_WDT
+	depends on SPL_DM_GPIO
+	depends on SPL_OF_REAL
+	default WDT_GPIO
+	help
+	  Support for external watchdog fed by toggling a gpio in SPL.
+
 config WDT_MAX6370
 	bool "MAX6370 watchdog timer support"
 	depends on WDT