boards: cdns: adopt new zephyr:board directive and role

This updates the documentation of all the Cadence boards
to use the new `zephyr:board::` directive.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

boards/cdns/xt-sim/doc/index.rst

@@ -1,7 +1,4 @@
-.. _xt-sim:
-
-Xtensa simulator
-################
+.. zephyr:board:: xt-sim
 
 Overview
 ********
@@ -13,12 +10,6 @@ addition to a base ISA to tailor the processor for a particular application.
 
 For more information, see https://ip.cadence.com/ipportfolio/tensilica-ip/xtensa-customizable
 
-.. figure:: img/xt-sim.jpg
-     :align: center
-     :alt: Xtensa Xplorer (Eclipse base frontend for xt-sim)
-
-     Xtensa Xplorer
-
 Hardware
 ********