powerpc: B4420: Remove macro CONFIG_PPC_B4420

Replace CONFIG_PPC_B4420 with ARCH_B4420 in Kconfig and clean up
existing macros.
diff --git a/configs/B4420QDS_SPIFLASH_defconfig b/configs/B4420QDS_SPIFLASH_defconfig
index dc4109b..fcf683d 100644
--- a/configs/B4420QDS_SPIFLASH_defconfig
+++ b/configs/B4420QDS_SPIFLASH_defconfig
@@ -5,7 +5,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="PPC_B4420,RAMBOOT_PBL,SPIFLASH,SYS_TEXT_BASE=0xFFF40000"
+CONFIG_SYS_EXTRA_OPTIONS="RAMBOOT_PBL,SPIFLASH,SYS_TEXT_BASE=0xFFF40000"
 CONFIG_BOOTDELAY=10
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y