Even if the board-specific directory Makefile doesn't have any
targets, it still needs to exist.
This adds a minimal Makefile for the board/rockchip/evb_rk3128
directory and a evk-rk3128.c (as built-in.o needs to be built
for every directory that a Makefile gets run for).
Fixes: c7a6866 ("rockchip: rk3128: add evb-rk3128 support")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
