spi: docs: convert to ReST and add it to the kABI bookset

While there's one file there with briefily describes the uAPI,
the documentation was written just like most subsystems: focused
on kernel developers. So, add it together with driver-api books.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # for iio
Signed-off-by: Jonathan Corbet <corbet@lwn.net>