ARM: dt: change .dtb build rules to build in dts directory

The current rules have the .dtb files build in a different directory
from the .dts files. The only reason for this is that it was what
PowerPC has done historically. This patch changes ARM to use the generic
dtb rule which builds .dtb files in the same directory as the source .dts.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Olof Johansson <olof@lixom.net>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
[swarren: added rm command for old stale .dtb files]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>