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>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index 909f991..ab38ced 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -33,6 +33,7 @@
    pci/index
    spi
    i2c
+   i3c/index
    hsi
    edac
    scsi