docs: nvmem: generate kernel-doc API documentation

This is useful on its own, and it also enables hyperlink generation for
functions mentioned in ReST documentation.
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231201-nvmem-docs-kerneldoc-v1-1-3e8f2b706ce6@bootlin.com

Documentation/driver-api/nvmem.rst

@@ -200,3 +200,9 @@ and let you add cells dynamically.
 Another use case for layouts is the post processing of cells. With layouts,
 it is possible to associate a custom post processing hook to a cell. It
 even possible to add this hook to cells not created by the layout itself.
+
+9. Internal kernel API
+======================
+
+.. kernel-doc:: drivers/nvmem/core.c
+   :export: