boards: others: neorv32: mention the GPTMR as being supported

Mention the NEORV32 General Purpose Timer (GPTMR) as supported and add
instruction on how to enable it in the devicetree.

Signed-off-by: Henrik Brix Andersen <[email protected]>

Author: henrikbrixandersen

boards/others/neorv32/doc/index.rst

@@ -111,6 +111,14 @@ The True Random-Number Generator (TRNG) of the NEORV32 is supported, but disable
 NEORV32 SoC implementations supporting the TRNG, support can be enabled by setting the ``status``
 property of the ``trng`` devicetree node to ``okay``.
 
+General Purpose Timer
+=====================
+
+The General Purpose Timer (GPTMR) of the NEORV32 is supported, but disabled by default. For NEORV32
+SoC implementations supporting the GPTMR, support can be enabled by setting the ``status`` property
+of the ``gptmr`` devicetree node to ``okay`` and selecting the desired GPTMR clock prescaler using
+the node's ``prescaler`` property.
+
 Programming and Debugging
 *************************