Convert CONFIG_SYS_DEVICE_NULLDEV to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_DEVICE_NULLDEV

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/M5249EVB_defconfig b/configs/M5249EVB_defconfig
index d4871c6..8f8a4a6 100644
--- a/configs/M5249EVB_defconfig
+++ b/configs/M5249EVB_defconfig
@@ -5,6 +5,7 @@
 CONFIG_TARGET_M5249EVB=y
 CONFIG_DEFAULT_DEVICE_TREE="M5249EVB"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
+CONFIG_SYS_DEVICE_NULLDEV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTOBOOT is not set