MAINTAINERS: Add file patterns for ipmi device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>

MAINTAINERS

@@ -7635,6 +7635,7 @@ M:	Corey Minyard <minyard@acm.org>
 L:	openipmi-developer@lists.sourceforge.net (moderated for non-subscribers)
 W:	http://openipmi.sourceforge.net/
 S:	Supported
+F:	Documentation/devicetree/bindings/ipmi/
 F:	Documentation/IPMI.txt
 F:	drivers/char/ipmi/
 F:	include/linux/ipmi*