docs: driver-api: Add I3C documentation

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

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>