powerpc: C29X: Move CONFIG_PPC_C29X to Kconfig option

Replace CONFIG_PPC_C29X with ARCH_C29X 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 4fd2ea6..640c846 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -33,6 +33,7 @@
 
 config TARGET_C29XPCIE
 	bool "Support C29XPCIE"
+	select ARCH_C29X
 	select SUPPORT_SPL
 	select SUPPORT_TPL
 	select PHYS_64BIT
@@ -185,6 +186,9 @@
 config ARCH_BSC9132
 	bool
 
+config ARCH_C29X
+	bool
+
 config ARCH_MPC8544
 	bool