docs: split up serial-interfaces.rst

It never made sense to keep these documents together; move each into its
own file.

Drop the section numbering on hsi.txt on its way to its own file.

Suggested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index b50c410..8e259c5d 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -20,5 +20,7 @@
    sound
    frame-buffer
    input
-   serial-interfaces
+   spi
+   i2c
+   hsi
    miscellaneous