powerpc: B4420QDS: Split from B4860QDS in Kconfig

Use TARGET_B4420QDS to simplify Kconfig options.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/b4860qds/Kconfig b/board/freescale/b4860qds/Kconfig
index c7aab75..01d68e1 100644
--- a/board/freescale/b4860qds/Kconfig
+++ b/board/freescale/b4860qds/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_B4860QDS
+if TARGET_B4860QDS || TARGET_B4420QDS
 
 config SYS_BOARD
 	default "b4860qds"