powerpc: P5020: Remove macro CONFIG_PPC_P5020

Replace CONFIG_PPC_P5020 with ARCH_P5020 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 011aec6..496b845 100644
--- a/arch/powerpc/include/asm/config_mpc85xx.h
+++ b/arch/powerpc/include/asm/config_mpc85xx.h
@@ -429,7 +429,7 @@
 #define CONFIG_SYS_FSL_ERRATUM_A007075
 #define CONFIG_SYS_FSL_A004447_SVR_REV	0x20
 
-#elif defined(CONFIG_PPC_P5020) /* also supports P5010 */
+#elif defined(CONFIG_ARCH_P5020) /* also supports P5010 */
 #define CONFIG_SYS_PPC64		/* 64-bit core */
 #define CONFIG_SYS_FSL_QORIQ_CHASSIS1
 #define CONFIG_FSL_CORENET		/* Freescale CoreNet platform */