evb-rk3399: config: set emmc as default boot dev

rk3399 has two mmc dev controller:
mmc 0: SD card;
mmc 1: EMMC

U-Boot will scan the mmc boot device configured by CONFIG_SYS_MMC_ENV_DEV,
since evb has emmc on board, let's set the EMMC as default.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>