ARM: 7520/1: Build dtb files in all target

Tools like kisskb are good at finding build regressions in the kernel
sources. However, regressions in the DT desscriptions are not found,
because generally these build systems don't build the DT binary blobs.

Extend the ARM all target to build all enabled DTB files.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>