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>

arch/arm/boot/dts/Makefile

@@ -506,6 +506,7 @@ dtb-$(CONFIG_ARCH_ROCKCHIP) += \
 	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 \