Add support for Xilinx CAN FD core.

The major difference from the previously supported cores is that there
are TX mailboxes instead of a TX FIFO and the RX FIFO access method is
different.

We only transmit one frame at a time to prevent the HW from reordering
frames (it uses CAN ID priority order).

Support for CAN FD protocol is not added yet.

v2: Removed unnecessary "rx-mode" DT property and wrapped some long
    lines.
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>