arm64: dts: imx8mm-phg: Add LVDS compatible string

The imx8mm-phg board has an AUO G084SN05 V9 8.4" 800x600 LVDS panel.

Improve the devicetree description by passing the LVDS compatible
string to fix the following dt-schema warning:

imx8mm-phg.dtb: panel: compatible:0: 'panel-lvds' is not one of
['admatec,9904379f', 'auo,b101ew05', 'auo,g084sn05',
'chunghwa,claa070wp03xg','edt,etml0700z9ndha', 'hannstar,hsd101pww2',
'hydis,hv070wx2-1e0', 'jenson,bl-jt60050-01a', 'tbs,a711-panel']
...

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>