[] [Enable eMMC HS400 mode]

[Description]
Add eMMC HS400 mode for improving IO performance.

[Release-log]
N/A

Change-Id: I9c4d9ed7cb0a5bb4deefdaa6f9d7d290115378f7
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/4989108
3 files changed