ARM: shmobile: r8a7791 Koelsch DT reference C bits

Add DT reference support for the r8a7791 Koelsch board.

This board support file will be used together with common
clocks and multiplatform in the future.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>