docs: mmc: move it to the driver-api

Most of the stuff here is related to the kAPI.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index d6bf4a3..25f85d3 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -45,6 +45,7 @@
    miscellaneous
    mei/index
    mtd/index
+   mmc/index
    nvdimm/index
    w1
    rapidio/index