rockchip: rk3588-toybrick: Add .dtb-file entry in Makefile

Add Rockchip Toybrick TB-RK3588X .dtb-file entry in Makefile.

Fixes: 9fdd9a54 ("board: rockchip: add Rockchip Toybrick TB-RK3588X board")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>