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/cpu.c b/arch/powerpc/cpu/mpc85xx/cpu.c
index 268429b..d180c73 100644
--- a/arch/powerpc/cpu/mpc85xx/cpu.c
+++ b/arch/powerpc/cpu/mpc85xx/cpu.c
@@ -404,7 +404,7 @@
 phys_size_t initdram(int board_type)
 {
 #if defined(CONFIG_SPD_EEPROM) || defined(CONFIG_DDR_SPD) || \
-	defined(CONFIG_QEMU_E500)
+	defined(CONFIG_ARCH_QEMU_E500)
 	return fsl_ddr_sdram_size();
 #else
 	return (phys_size_t)CONFIG_SYS_SDRAM_SIZE * 1024 * 1024;