decode device tree file name by board revison resisors (e.g. omap3-gta04a3.dtb)

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
added missing files for gta04b7 and gta04b8

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>