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