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>