ppc: mpc8xx: Drop -mstring from PLATFORM_CPPFLAGS

This has not been supported by toolchains for some time and has been
putting out a warning.  Drop this.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/arch/powerpc/cpu/mpc8xx/config.mk b/arch/powerpc/cpu/mpc8xx/config.mk
index 00b7ed5..5a64665 100644
--- a/arch/powerpc/cpu/mpc8xx/config.mk
+++ b/arch/powerpc/cpu/mpc8xx/config.mk
@@ -3,4 +3,4 @@
 # (C) Copyright 2000-2010
 # Wolfgang Denk, DENX Software Engineering, wd@denx.de.
 
-PLATFORM_CPPFLAGS += -mstring -mcpu=860 -msoft-float
+PLATFORM_CPPFLAGS += -mcpu=860 -msoft-float