[][openwrt][mt7988][audio][Add si3218x-spi for si3218x slic driver test]

[Description]
Add si3218x-spi for si3218x slic driver test, and must sign NDA with
Silicon Labs to obtain si3218x slic driver.

For Silicon Labs slic driver further information:
Contact window: EDOM TECHNOLOGY CO., LTD
Tel: 02-2657-8811
Web: www.edom.com.tw

[Release-log]
N/A

Change-Id: I071b589ace650b0214dbe893d9048668f94a2fe4
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8249671
1 file changed