powerpc: mpc5xx: remove redundant CONFIG_5xx definition

We do not have to define CONFIG_5xx in a source file
because it is defined in arch/powerpc/cpu/mpc5xx/config.mk.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>

arch/powerpc/cpu/mpc5xx/start.S

@@ -19,8 +19,6 @@
 #include <mpc5xx.h>
 #include <version.h>
 
-#define CONFIG_5xx 1		/* needed for Linux kernel header files */
-
 #include <ppc_asm.tmpl>
 #include <ppc_defs.h>