docs: Convert the regulator docbook to RST

A fairly straightforward conversion to RST; the document is then added to
the driver-api manual.

Of course, this document has seen no substantive changes since 2008, so
chances are it needs work in other areas as well.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>