imx8*_venice_defconfig: configure default MMC env device

When booting from USB/SDP mmc_get_env_dev() returns
CONFIG_SYS_MMC_ENV_DEV as the MMC env device (while booting from MMC
will call board_mmc_get_env_dev() to get this). Configure
CONFIG_SYS_MMC_ENV_DEV for SDHC3 (devno=2) as all Gateworks Venice
boards use SDHC3 as eMMC so that persistent env works when booting from
USB/SDP.

Reviewed-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
diff --git a/configs/imx8mp_venice_defconfig b/configs/imx8mp_venice_defconfig
index f3f98aa..84e708a 100644
--- a/configs/imx8mp_venice_defconfig
+++ b/configs/imx8mp_venice_defconfig
@@ -76,6 +76,7 @@
 CONFIG_OF_LIST="imx8mp-venice imx8mp-venice-gw74xx"
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
+CONFIG_SYS_MMC_ENV_DEV=2
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_IP_DEFRAG=y