docs: add a reset controller chapter to the driver API docs

Add initial reset controller API documentation. This is mostly intended
to describe the concepts to users of the consumer API, and to tie the
kerneldoc comments we already have into the driver API documentation.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Amjad Ouled-Ameur <aouledameur@baylibre.fr>
Link: https://lore.kernel.org/r/20201201115754.1713-1-p.zabel@pengutronix.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>