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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/nvmem.rst

···
 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: