Kconfig: Convert CONFIG_IMX_WATCHDOG to Kconfig

Move this option to Kconfig and tidy up the config file of eight
boards which use it.

Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
diff --git a/configs/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 2cb7164..92b2377 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -59,4 +59,5 @@
 CONFIG_USB_GADGET_PRODUCT_NUM=0xa4a5
 CONFIG_CI_UDC=y
 CONFIG_USB_GADGET_DOWNLOAD=y
+CONFIG_IMX_WATCHDOG=y
 CONFIG_OF_LIBFDT=y