powerpc: MPC8555: Remove macro CONFIG_MPC8555

Replace CONFIG_MPC8555 with ARCH_MPC8555 in Kconfig and clean up existing
macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 8b08ee2..c58f76f 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -76,6 +76,7 @@
 
 config TARGET_MPC8555CDS
 	bool "Support MPC8555CDS"
+	select ARCH_MPC8555
 
 config TARGET_MPC8560ADS
 	bool "Support MPC8560ADS"
@@ -207,6 +208,9 @@
 config ARCH_MPC8548
 	bool
 
+config ARCH_MPC8555
+	bool
+
 source "board/freescale/b4860qds/Kconfig"
 source "board/freescale/bsc9131rdb/Kconfig"
 source "board/freescale/bsc9132qds/Kconfig"