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/brppt1_spi_defconfig b/configs/brppt1_spi_defconfig
index cc6544b..08c7378 100644
--- a/configs/brppt1_spi_defconfig
+++ b/configs/brppt1_spi_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
+CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,SPI_BOOT,EMMC_BOOT"
 CONFIG_SPI_BOOT=y