docs: move mailbox.txt to driver-api and rename it

This file is already at the ReST format. Move it to
driver-api and rename it.
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/03e40c31b86c1f4fd3597bf4bfb8346901286bab.1592918949.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

@@ -48,6 +48,7 @@ available subsections can be seen below.
    scsi
    libata
    target
+   mailbox
    mtdnand
    miscellaneous
    mei/index