Kill of the defconfig bits now when the marzen-reference board code is gone.

Based on work by Magnus Damm.
Acked-by: NMagnus Damm <damm@opensource.se>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>