kconfig: delete redundant CONFIG_${ARCH} definition

CONFIG_${ARCH} is defined by Kconfig.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/config.mk b/arch/mips/config.mk
index f4a234a..a2d07af 100644
--- a/arch/mips/config.mk
+++ b/arch/mips/config.mk
@@ -25,7 +25,7 @@
 # Default to EB if no endianess is configured
 ENDIANNESS ?= -EB
 
-PLATFORM_CPPFLAGS += -DCONFIG_MIPS -D__MIPS__
+PLATFORM_CPPFLAGS += -D__MIPS__
 
 __HAVE_ARCH_GENERIC_BOARD := y