ARM: dts: rockchip: add rk3288-popmetal board to dtb list

The popmetal board was not included in the list of Rockchip boards,
so was only built when explicitly called with make rk3288-popmetal.dtb
but not in a generic make dtbs, so add the missing entry.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile
index 388bd7d..301b25e 100644
--- a/arch/arm/boot/dts/Makefile
+++ b/arch/arm/boot/dts/Makefile
@@ -506,6 +506,7 @@
 	rk3288-evb-rk808.dtb \
 	rk3288-firefly-beta.dtb \
 	rk3288-firefly.dtb \
+	rk3288-popmetal.dtb \
 	rk3288-r89.dtb \
 	rk3288-rock2-square.dtb \
 	rk3288-veyron-jaq.dtb \