docs: ctucanfd: Use 'kernel-figure' directive instead of 'figure'

Two issues were observed in the ReST doc added by commit c3a0addefb
("docs: ctucanfd: CTU CAN FD open-source IP core documentation.")
with Sphinx versions 2.4.4 and 4.5.0.

The plain "figure" directive broke "make pdfdocs" due to a missing
PDF figure.  For conversion of SVG -> PDF to work, the "kernel-figure"
directive, which is an extension for kernel documentation, should
be used instead.

The directive of "code:: raw" causes a warning from both
"make htmldocs" and "make pdfdocs", which reads:

    [...]/can/ctu/ctucanfd-driver.rst:75: WARNING: Pygments lexer name
    'raw' is not known

A plain literal-block marker should suffice where no syntax
highlighting is intended.

Fix the issues by using suitable directive and marker.

Fixes: c3a0addefb ("docs: ctucanfd: CTU CAN FD open-source IP core documentation.")
Link: https://lore.kernel.org/all/5986752a-1c2a-5d64-f91d-58b1e6decd17@gmail.com
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Cc: Martin Jerabek <martin.jerabek01@gmail.com>
Cc: Ondrej Ille <ondrej.ille@gmail.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/networking/device_drivers/can/ctu/ctucanfd-driver.rst

@@ -72,7 +72,7 @@ it is reachable (on which bus it resides) and its configuration –
 registers address, interrupts and so on. An example of such a device
 tree is given in .
 
-.. code:: raw
+::
 
            / {
                /* ... */
@@ -451,7 +451,7 @@ the FIFO is maintained, together with priority rotation, is depicted in
 
 |
 
-.. figure:: fsm_txt_buffer_user.svg
+.. kernel-figure:: fsm_txt_buffer_user.svg
 
    TX Buffer states with possible transitions