powerpc: B4420: Remove macro CONFIG_PPC_B4420

Replace CONFIG_PPC_B4420 with ARCH_B4420 in Kconfig and clean up
existing macros.
diff --git a/board/freescale/b4860qds/eth_b4860qds.c b/board/freescale/b4860qds/eth_b4860qds.c
index 164ec0a..89a1883 100644
--- a/board/freescale/b4860qds/eth_b4860qds.c
+++ b/board/freescale/b4860qds/eth_b4860qds.c
@@ -213,7 +213,7 @@
 		fm_info_set_phy_address(FM1_DTSEC6,
 				CONFIG_SYS_FM1_ONBOARD_PHY2_ADDR);
 		break;
-#ifdef CONFIG_PPC_B4420
+#ifdef CONFIG_ARCH_B4420
 	case 0x17:
 	case 0x18:
 		/* Serdes 1: A-D SGMII, Configuring on board dual SGMII Phy */