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/mpc512x/config.mk b/arch/powerpc/cpu/mpc512x/config.mk
index 03759e6..5bf1b2a 100644
--- a/arch/powerpc/cpu/mpc512x/config.mk
+++ b/arch/powerpc/cpu/mpc512x/config.mk
@@ -4,4 +4,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_CPPFLAGS += -DCONFIG_MPC512X -DCONFIG_E300 -msoft-float -mcpu=603e
+PLATFORM_CPPFLAGS += -DCONFIG_E300 -msoft-float -mcpu=603e