watchdog: Split WDT from SPL_WDT

Use CONFIG_IS_ENABLED(WDT) to permit use of WDT in SPL without DM,
while the full U-Boot can use rich DM/DT WDT driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>
Tested-by: Suniel Mahesh <sunil.m@techveda.org>
diff --git a/common/spl/spl.c b/common/spl/spl.c
index d5e3f68..c182705 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -603,7 +603,7 @@
 	spl_board_init();
 #endif
 
-#if defined(CONFIG_SPL_WATCHDOG_SUPPORT) && defined(CONFIG_WDT)
+#if defined(CONFIG_SPL_WATCHDOG_SUPPORT) && CONFIG_IS_ENABLED(WDT)
 	initr_watchdog();
 #endif