powerpc: mpc86xx: move CONFIG_MPC86xx definition to CPU config.mk

Define CONFIG_MPC86xx in arch/powerpc/cpu/mpc86xx/config.mk
because all target boards with mpc86xx cpu define it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/powerpc/cpu/mpc86xx/config.mk b/arch/powerpc/cpu/mpc86xx/config.mk
index 69a0b96..4c7235f 100644
--- a/arch/powerpc/cpu/mpc86xx/config.mk
+++ b/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 += -DCONFIG_MPC86xx -mstring -maltivec -mabi=altivec -msoft-float