powerpc/86xx: Pass -mcpu=7400 to GCC

Without this, GCC uses the toolchain default, which may be incompatible
with -maltivec.

Signed-off-by: Scott Wood <oss@buserror.net>
Reviewed-by: York Sun <york.sun@nxp.com>

arch/powerpc/cpu/mpc86xx/config.mk

@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_CPPFLAGS += -mstring -maltivec -mabi=altivec -msoft-float
+PLATFORM_CPPFLAGS += -mcpu=7400 -mstring -maltivec -mabi=altivec -msoft-float