"simple-panel" is a Linux driver and has never been an accepted upstream
compatible string, so remove it.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200117230845.25190-1-robh@kernel.orgSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>