ARM: 7152/1: distclean: Remove generated .dtb files

The patch 'arm/dt: Add dtb make rule' adds support to
create a .dtb file. But this is never removed afterwards.
Remove the generated .dtb file if 'distclean' is called.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Shawn Guo <shawn.guo@freescale.com>
Cc: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>