ARM: shmobile: koelsch: Remove reference board code

The Koelsch board is supported by the r8a7791 generic DT platform
definition. Remove the board-specific definition along with its board
file.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>