powerpc: P5040: Remove macro CONFIG_P5040

Replace CONFIG_P5040 with ARCH_P5040 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 808144e..9aef492 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -56,6 +56,7 @@
 config TARGET_P5040DS
 	bool "Support P5040DS"
 	select PHYS_64BIT
+	select ARCH_P5040
 
 config TARGET_MPC8536DS
 	bool "Support MPC8536DS"
@@ -328,6 +329,9 @@
 config ARCH_P5020
 	bool
 
+config ARCH_P5040
+	bool
+
 source "board/freescale/b4860qds/Kconfig"
 source "board/freescale/bsc9131rdb/Kconfig"
 source "board/freescale/bsc9132qds/Kconfig"