mx7ulp_evk_plugin: Disable CONFIG_NET

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>