can: Kconfig: CAN_AT91 depends on ARM

SAMA5D3 devices also embed CAN feature. Moreover if we want to produce a single
kernel image it is not useful to be too restrictive.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>