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_SECURE_BOOT_defconfig b/configs/B4860QDS_SECURE_BOOT_defconfig
index 2923547..53c122c 100644
--- a/configs/B4860QDS_SECURE_BOOT_defconfig
+++ b/configs/B4860QDS_SECURE_BOOT_defconfig
@@ -6,7 +6,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="PPC_B4860,SECURE_BOOT"
+CONFIG_SYS_EXTRA_OPTIONS="SECURE_BOOT"
 CONFIG_BOOTDELAY=10
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y