ARM: dts: stm32: drop invalid simple-panel compatible on stm32mp157c-lxa

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

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>