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/immap_85xx.h b/arch/powerpc/include/asm/immap_85xx.h
index 83f8b8f..759671a 100644
--- a/arch/powerpc/include/asm/immap_85xx.h
+++ b/arch/powerpc/include/asm/immap_85xx.h
@@ -1857,7 +1857,7 @@
 #define FSL_CORENET_RCWSR11_EC2_USB2			0x00100000
 #endif
 #if defined(CONFIG_ARCH_P2041) || \
-	defined(CONFIG_ARCH_P3041) || defined(CONFIG_PPC_P5020)
+	defined(CONFIG_ARCH_P3041) || defined(CONFIG_ARCH_P5020)
 #define FSL_CORENET_RCWSR11_EC1_FM1_DTSEC4_RGMII	0x00000000
 #define FSL_CORENET_RCWSR11_EC1_FM1_DTSEC4_MII		0x00800000
 #define FSL_CORENET_RCWSR11_EC1_FM1_DTSEC4_NONE		0x00c00000