driver-api: create an edac.rst file with EDAC documentation

Currently, there's no device driver documentation for the EDAC
subsystem at the driver-api book. Fill in the blanks for the
structures and functions that misses documentation, uniform
the word on the existing ones, and add a new edac.rst file at
driver-api, in order to document the EDAC subsystem.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>