powerpc: B4860: Remove macro CONFIG_PPC_B4860

Replace CONFIG_PPC_B4860 with ARCH_B4860 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 31a074e..962ec2b 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -23,6 +23,7 @@
 
 config TARGET_B4860QDS
 	bool "Support B4860QDS"
+	select ARCH_B4860
 	select SUPPORT_SPL
 	select PHYS_64BIT
 
@@ -256,6 +257,9 @@
 
 endchoice
 
+config ARCH_B4860
+	bool
+
 config ARCH_BSC9131
 	bool