[][Add trng device node for mt7981]

[Description]
Add trng device mode.

[Release-log]
N/A

Change-Id: Iba02f392920b2cb9edf5c2c63d8614391da79545
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5379897
1 file changed