ppc4xx: Add common ppc4xx linker script

This linker script can be used by all PPC4xx platforms. It works for
PPC405 and PPC440 platforms. Boards which need a board specific linker
script can override this default linker script in board/*/config.mk.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/cpu/ppc4xx/config.mk b/cpu/ppc4xx/config.mk
index 00ad39b..979004b 100644
--- a/cpu/ppc4xx/config.mk
+++ b/cpu/ppc4xx/config.mk
@@ -32,3 +32,6 @@
 else
 PLATFORM_CPPFLAGS += -Wa,-m405 -mcpu=405
 endif
+
+# Use default linker script.  Board port can override in board/*/config.mk
+LDSCRIPT := $(SRCTREE)/cpu/ppc4xx/u-boot.lds