mx7ulp_evk_plugin: Disable CONFIG_NET

Currently the following build warning is seen:

===================== WARNING ======================
This board does not use CONFIG_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
===================================================

Since the mx7ulp-evk board does not have networking support, explicitly
disable networking.

Signed-off-by: Fabio Estevam <festevam@gmail.com>

configs/mx7ulp_evk_plugin_defconfig

@@ -17,6 +17,7 @@ CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="imx7ulp-evk"
 CONFIG_ENV_IS_IN_MMC=y
+# CONFIG_NET is not set
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DM=y
 CONFIG_DM_GPIO=y