dt-bindings: arm: Remove leftover axentia.txt

The bindings described in axentia.txt are already covered by
atmel-at91.yaml, so remove the file.

Cc: Peter Rosin <peda@axentia.se>
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>