Remove board support for the r8a7740 based Bonito board

The r8a7740 SoC support code is still kept around since it
is in use by the Armadillo800eva board which is basically a more
recent board where the design is based on Bonito.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>