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>

include/asm-generic/global_data.h

@@ -137,7 +137,7 @@ typedef struct global_data {
 #if defined(CONFIG_TRANSLATION_OFFSET)
 	fdt_addr_t translation_offset;	/* optional translation offset */
 #endif
-#if defined(CONFIG_WDT)
+#if CONFIG_IS_ENABLED(WDT)
 	struct udevice *watchdog_dev;
 #endif
 } gd_t;