powerpc: B4860: Remove macro CONFIG_PPC_B4860

Replace CONFIG_PPC_B4860 with ARCH_B4860 in Kconfig and clean up
existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/configs/B4860QDS_SPIFLASH_defconfig b/configs/B4860QDS_SPIFLASH_defconfig
index 80e729e..94a07fa 100644
--- a/configs/B4860QDS_SPIFLASH_defconfig
+++ b/configs/B4860QDS_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_B4860,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