at91_can: add driver for Atmel's CAN controller on AT91SAM9263

This patch add the driver for the SoC CAN controller in Atmel's
AT91SAM9263.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>