Add compatible string and new attributes to support the Xilinx CAN FD
core.

Unlike the previously documented Xilinx CAN cores, the CAN FD core has
TX mailboxes instead of TX FIFO, and optionally RX mailboxes instead of
RX FIFO (selected at core generation time, not switchable at runtime).
Add "tx-mailbox-count" and "rx-mailbox-count" to specify the mailbox
counts instead of reusing "tx-fifo-depth" and "rx-fifo-depth".

The RX FIFO depth is constant 32, but allow it to be specified via
"rx-fifo-depth" to match DT usage with Zynq CAN (which has constant RX
FIFO of depth of 64).

v2: Remove unnecessary "rx-mode" DT property.
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>