Add compilation support for dtbs using 'make dtbs'

Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Simon Horman <horms@verge.net.au>
Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Signed-off-by: Simon Horman <horms@verge.net.au>