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/configs/B4420QDS_defconfig b/configs/B4420QDS_defconfig
index 406241a..db1b8ac 100644
--- a/configs/B4420QDS_defconfig
+++ b/configs/B4420QDS_defconfig
@@ -1,6 +1,6 @@
 CONFIG_PPC=y
 CONFIG_MPC85xx=y
-CONFIG_TARGET_B4860QDS=y
+CONFIG_TARGET_B4420QDS=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y