docs: Pull the HSI documentation together

The HSI subsystem documentation was split across hsi.txt and the
device-drivers docbook.  Now that the latter has been converted to Sphinx,
pull in the HSI document so that it's all in one place.

Acked-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/MAINTAINERS b/MAINTAINERS
index 8107235..769a2fd 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -5606,7 +5606,7 @@
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/sre/linux-hsi.git
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-bus-hsi
-F:	Documentation/hsi.txt
+F:	Documentation/device-drivers/serial-interfaces.rst
 F:	drivers/hsi/
 F:	include/linux/hsi/
 F:	include/uapi/linux/hsi/