ARM: shmobile: Remove Genmai reference DTS

Now that the DTS file r7s72100-genmai.dts can be used with
board-genmai.c and board-genmai-reference.c, proceed with removing
r7s72100-genmai-reference.dts.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>