board: rk3288: add more DT files to MAINTAINERS

A number of rk3229/rk3288 DT files are synced from Linux.
Add a maintainer to look after them and to help with
review and testing.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/board/chipspark/popmetal_rk3288/MAINTAINERS b/board/chipspark/popmetal_rk3288/MAINTAINERS
index 1a6a1bb..e12f128 100644
--- a/board/chipspark/popmetal_rk3288/MAINTAINERS
+++ b/board/chipspark/popmetal_rk3288/MAINTAINERS
@@ -1,6 +1,8 @@
 POPMETAL-RK3288
 M:	Lin Huang <hl@rock-chips.com>
 S:	Maintained
+F:	arch/arm/dts/rk3288-popmetal.dts
+F:	arch/arm/dts/rk3288-popmetal-u-boot.dtsi
 F:	board/chipspark/popmetal_rk3288
 F:	include/configs/popmetal_rk3288.h
 F:	configs/popmetal-rk3288_defconfig