powerpc: BSC9131/2: Move CONFIG_BSC9131/2 to Kconfig options

Replace CONFIG_BSC9131, CONFIG_BSC9132 with ARCH_BSC9131, ARCH_BSC9132
Kconfig options.

Also drop #ifdef in BSC9131RDB.h since it is redundant.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 5ea02b5..4fd2ea6 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -23,10 +23,12 @@
 
 config TARGET_BSC9131RDB
 	bool "Support BSC9131RDB"
+	select ARCH_BSC9131
 	select SUPPORT_SPL
 
 config TARGET_BSC9132QDS
 	bool "Support BSC9132QDS"
+	select ARCH_BSC9132
 	select SUPPORT_SPL
 
 config TARGET_C29XPCIE
@@ -177,6 +179,12 @@
 
 endchoice
 
+config ARCH_BSC9131
+	bool
+
+config ARCH_BSC9132
+	bool
+
 config ARCH_MPC8544
 	bool