powerpc: remove redundant CPU definition

CONFIG_${CPU} is defined by Kconfig.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Stefan Roese <sr@denx.de>
diff --git a/arch/powerpc/cpu/74xx_7xx/config.mk b/arch/powerpc/cpu/74xx_7xx/config.mk
index 96812a0..4cd1a26 100644
--- a/arch/powerpc/cpu/74xx_7xx/config.mk
+++ b/arch/powerpc/cpu/74xx_7xx/config.mk
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_CPPFLAGS += -DCONFIG_74xx_7xx -mstring
+PLATFORM_CPPFLAGS += -mstring