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/pepper_defconfig b/configs/pepper_defconfig
index 81289b9..f297a94 100644
--- a/configs/pepper_defconfig
+++ b/configs/pepper_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y