can: xilinx_can: add support for Xilinx CAN FD core

can: xilinx_can: add support for Xilinx CAN FD core

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 <[email protected]>
Cc: Michal Simek <[email protected]>
Signed-off-by: Marc Kleine-Budde <[email protected]>