iot_dk: Disable networking support

There's no Ethernet controller on the board so no point in having
networking support. This also saves us 5.5 kB of precious memory.

| # bloat-o-meter u-boot.net u-boot.no_net_regex | tail -1
| Total: Before=127892, After=122334, chg -4.35%

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

configs/iot_devkit_defconfig

@@ -18,7 +18,6 @@ CONFIG_SYS_PROMPT="IoTDK# "
 # CONFIG_CMD_LOADS is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y
-# CONFIG_CMD_NET is not set
 CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y
@@ -26,6 +25,7 @@ CONFIG_DEFAULT_DEVICE_TREE="iot_devkit"
 CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_FAT_INTERFACE="mmc"
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
+# CONFIG_NET is not set
 CONFIG_DM=y
 CONFIG_MMC=y
 CONFIG_MMC_DW=y