powerpc: P2020: Remove macro CONFIG_P2020

Replace CONFIG_P2020 with ARCH_P2020 in Kconfig and clean up
existing macros.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/arch/powerpc/include/asm/config_mpc85xx.h b/arch/powerpc/include/asm/config_mpc85xx.h
index 6874b54..814a9fb 100644
--- a/arch/powerpc/include/asm/config_mpc85xx.h
+++ b/arch/powerpc/include/asm/config_mpc85xx.h
@@ -295,7 +295,7 @@
 #define CONFIG_SYS_FSL_ERRATUM_A004508
 #define CONFIG_SYS_FSL_ERRATUM_A005125
 
-#elif defined(CONFIG_P2020)
+#elif defined(CONFIG_ARCH_P2020)
 #define CONFIG_MAX_CPUS			2
 #define CONFIG_SYS_FSL_NUM_LAWS		12
 #define CONFIG_SYS_PPC_E500_DEBUG_TLB	2