docs: nvmem: convert docs to ReST and rename to *.rst

docs: nvmem: convert docs to ReST and rename to *.rst

In order to be able to add it into a doc book, we need first
convert it to ReST.

The conversion is actually:
  - add blank lines and identation in order to identify paragraphs;
  - mark literal blocks;
  - adjust title markups.

While this is not part of any book, mark it as :orphan:, in order
to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>