The dt-binding for Mediatek's SPI NOR flash controller expects a mt8173
fallback compatible for mt8192, so add it in mt8192.dtsi.
The driver already sets custom data based on the mt8192 compatible, so
this fallback compatible won't be used and is added purely to suppress
the dt-binding warning.
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>