Documentation: Add SLIMbus summary

SLIMbus (Serial Low Power Interchip Media Bus) is a specification
developed by MIPI (Mobile Industry Processor Interface) alliance.
SLIMbus is a 2-wire implementation, which is used to communicate with
peripheral components like audio-codec.

The summary of SLIMbus and API is documented in the 'summary' file.

Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviwed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index d17a987..ef7c9e0 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -47,6 +47,7 @@
    gpio
    misc_devices
    dmaengine/index
+   slimbus
 
 .. only::  subproject and html