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/b4860qds_crossbar_con.h b/board/freescale/b4860qds/b4860qds_crossbar_con.h
index fcccb8f..901f8b0 100644
--- a/board/freescale/b4860qds/b4860qds_crossbar_con.h
+++ b/board/freescale/b4860qds/b4860qds_crossbar_con.h
@@ -28,7 +28,7 @@
 				{7, 8}, {9, 0}, {5, 14},
 				{4, 15}, {2, 12}, {12, 13} };
 
-#ifdef CONFIG_PPC_B4420
+#ifdef CONFIG_ARCH_B4420
 static int8_t vsc16_tx_sgmii_lane_cd[8][2] = { {5, 14}, {4, 15},
 		{-1, -1}, {-1, -1}, {-1, -1}, {-1, -1}, {-1, -1}, {-1, -1} };
 #endif
@@ -54,7 +54,7 @@
 				{7, 8}, {1, 9}, {14, 11},
 				{15, 10}, {13, 3}, {12, 12} };
 
-#ifdef CONFIG_PPC_B4420
+#ifdef CONFIG_ARCH_B4420
 static int8_t vsc16_rx_sgmii_lane_cd[8][2] = { {14, 11}, {15, 10},
 		{-1, -1}, {-1, -1}, {-1, -1}, {-1, -1}, {-1, -1}, {-1, -1} };
 #endif