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/gwventana_defconfig b/configs/gwventana_defconfig
index 77bb471..9183147 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x18000000
 CONFIG_DM_SERIAL=y
 CONFIG_FIT=y