ppc: xilinx-ppc405-generic

Fix device tree name:

+(xilinx-ppc405-generic) Device Tree Source is not correctly specified.
+(xilinx-ppc405-generic) Please define 'CONFIG_DEFAULT_DEVICE_TREE'
+(xilinx-ppc405-generic) or build with 'DEVICE_TREE=<device_tree>'
argument
+(xilinx-ppc405-generic) make[2]: ***
[arch/powerpc/dts/xilinx-ppc440-generic.dtb] Error 1
+(xilinx-ppc405-generic) make[1]: *** [dts] Error 2

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>