can: at91_can: add support for the AT91SAM9X5 SOCs

The AT91SAM9X5 SOCs have a similar CAN core, but they only have 8 compared
to 16 mailboxes on the AT91SAM9263 SOC. Another difference is that the bits
defining the state of the CAN core are cleared on read, thus the driver
has to derive the state by looking at the error counters.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>