ppc: pp440-generic: Simplify Makefile

As a result of the specific board removal, the Makefiles can be
simplified.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>

board/xilinx/ppc440-generic/Makefile

@@ -9,5 +9,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	+= ../../xilinx/ppc440-generic/xilinx_ppc440_generic.o
-extra-y 	+= ../../xilinx/ppc440-generic/init.o
+obj-y	+= xilinx_ppc440_generic.o
+extra-y += init.o