[][mt7986: Add emmc image]

[Description]
Add mmc image, enable related kernel options and dts.

[Release-log]
N/A

Change-Id: I1727e72b19c67f971d9fab00f12ef6456dccc70d
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/4632363
4 files changed