MAINTAINERS: add include/power/ to POWER

Add the related include files to the power MAINTAINERS entry.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 8dcce88..eb6289d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1245,6 +1245,7 @@
 S:	Maintained
 T:	git https://source.denx.de/u-boot/custodians/u-boot-pmic.git
 F:	drivers/power/
+F:	include/power/
 
 POWERPC
 M:	Wolfgang Denk <wd@denx.de>