powerpc: B4420: Remove macro CONFIG_PPC_B4420

Replace CONFIG_PPC_B4420 with ARCH_B4420 in Kconfig and clean up
existing macros.
diff --git a/include/configs/B4860QDS.h b/include/configs/B4860QDS.h
index a1468b8..cd4333f 100644
--- a/include/configs/B4860QDS.h
+++ b/include/configs/B4860QDS.h
@@ -73,7 +73,7 @@
 #define CONFIG_FSL_PCI_INIT		/* Use common FSL init code */
 #define CONFIG_SYS_PCI_64BIT		/* enable 64-bit PCI resources */
 
-#ifndef CONFIG_PPC_B4420
+#ifndef CONFIG_ARCH_B4420
 #define CONFIG_SYS_SRIO
 #define CONFIG_SRIO1			/* SRIO port 1 */
 #define CONFIG_SRIO2			/* SRIO port 2 */