imx8*_venice_defconfig: configure default MMC env device

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>