arm64: dts: imx8mq-mnt-reform2: drop invalid simple-panel compatible

"simple-panel" compatible is not documented and nothing in Linux kernel
binds to it:

  imx8mq-mnt-reform2.dtb: panel: compatible: ['innolux,n125hce-gn1', 'simple-panel'] is too long

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>