docs: driver-api: Add I3C documentation

Add the I3C documentation describing the protocol, the master driver API
and the device driver API.

Change-Id: I474f18bf8f904bb469ab10be482927927f8d16c7
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Git-commit: bd360e8175f4e07418a8427786dfaf9d42867e71
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Alok Chauhan <alokc@codeaurora.org>