MAINTAINERS: update HSI entry

Add git tree for hsi subsystem, update Sebastian Reichel's e-mail
address and add Documentation/hsi.txt as maintained file.
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>

MAINTAINERS

@@ -4203,9 +4203,11 @@ S:	Maintained
 F:	fs/hpfs/
 
 HSI SUBSYSTEM
-M:	Sebastian Reichel <sre@debian.org>
+M:	Sebastian Reichel <sre@kernel.org>
+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:	drivers/hsi/
 F:	include/linux/hsi/
 F:	include/uapi/linux/hsi/