powerpc: QEMU_E500: Remove macro CONFIG_QEMU_E500

Replace CONFIG_QEMU_E500 with ARCH_QEMU_E500 in Kconfig and
clean up existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Makefile b/arch/powerpc/cpu/mpc85xx/Makefile
index c19c3e7..0ff64a7 100644
--- a/arch/powerpc/cpu/mpc85xx/Makefile
+++ b/arch/powerpc/cpu/mpc85xx/Makefile
@@ -105,7 +105,7 @@
 obj-y	+= cpu_init.o
 obj-y	+= cpu_init_early.o
 obj-y	+= interrupts.o
-ifneq ($(CONFIG_QEMU_E500),y)
+ifneq ($(CONFIG_ARCH_QEMU_E500),y)
 obj-y	+= speed.o
 endif
 obj-y	+= tlb.o