ARM: shmobile: henninger: remove board DT

The Henninger low cost board was never sold and so  had  no  users outside
Renesas (and Cogent Embedded). Instead Renesas  has made the Porter  board
which has mostly similar hardware. Now that the Porter board support is on
par with Henninger, we can remove  the Henninger's device tree file.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>