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/arch/powerpc/cpu/mpc85xx/Kconfig b/arch/powerpc/cpu/mpc85xx/Kconfig
index 9aef492..31a074e 100644
--- a/arch/powerpc/cpu/mpc85xx/Kconfig
+++ b/arch/powerpc/cpu/mpc85xx/Kconfig
@@ -16,6 +16,11 @@
 	bool "Support socrates"
 	select ARCH_MPC8544
 
+config TARGET_B4420QDS
+	bool "Support B4420QDS"
+	select SUPPORT_SPL
+	select PHYS_64BIT
+
 config TARGET_B4860QDS
 	bool "Support B4860QDS"
 	select SUPPORT_SPL