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/Kconfig b/common/spl/Kconfig
index 5978fb2..81bd908 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -1061,6 +1061,7 @@
 
 config SPL_WATCHDOG_SUPPORT
 	bool "Support watchdog drivers"
+	imply SPL_WDT if !HW_WATCHDOG
 	help
 	  Enable support for watchdog drivers in SPL. A watchdog is
 	  typically a hardware peripheral which can reset the system when it