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>