Convert CONFIG_SPL_WATCHDOG_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/zc5202_defconfig b/configs/zc5202_defconfig
index c0f3285..8d3f1df 100644
--- a/configs/zc5202_defconfig
+++ b/configs/zc5202_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6Q"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y