PowerPC: merge commonly-defined flags

PLATFORM_RELFLAGS += -meabi
PLATFORM_CPPFLAGS += -ffixed-r2
were defined in all arch/powerpc/${CPU}/config.mk.

This commit moves them to arch/powerpc/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/arch/powerpc/cpu/mpc5xx/config.mk b/arch/powerpc/cpu/mpc5xx/config.mk
index b33f17a..31e2dc9 100644
--- a/arch/powerpc/cpu/mpc5xx/config.mk
+++ b/arch/powerpc/cpu/mpc5xx/config.mk
@@ -5,6 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-PLATFORM_RELFLAGS +=	-meabi
-
-PLATFORM_CPPFLAGS +=	-DCONFIG_5xx -ffixed-r2 -mpowerpc -msoft-float
+PLATFORM_CPPFLAGS += -DCONFIG_5xx -mpowerpc -msoft-float