ARM: shmobile: marzen-reference: Remove C board code

The generic r8a7779 machine vector is now feature-wise equivalent
to the Marzen-reference case, so simply remove the Marzen C board
code to fall over on the generic r8a7779 machine vector.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>