powerpc: MPC8536: Move CONFIG_MPC8536 to Kconfig option

Replace CONFIG_MPC8536 with ARCH_MPC8536 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 cef9da0..1ae5af1 100644
--- a/arch/powerpc/include/asm/immap_85xx.h
+++ b/arch/powerpc/include/asm/immap_85xx.h
@@ -2120,7 +2120,7 @@
 #else
 typedef struct ccsr_gur {
 	u32	porpllsr;	/* POR PLL ratio status */
-#ifdef CONFIG_MPC8536
+#ifdef CONFIG_ARCH_MPC8536
 #define MPC85xx_PORPLLSR_DDR_RATIO	0x3e000000
 #define MPC85xx_PORPLLSR_DDR_RATIO_SHIFT	25
 #elif defined(CONFIG_ARCH_C29X)
@@ -2203,7 +2203,7 @@
 	u8	res1[8];
 	u32	gpporcr;	/* General-purpose POR configuration */
 	u8	res2[12];
-#if defined(CONFIG_MPC8536)
+#if defined(CONFIG_ARCH_MPC8536)
 	u32	gencfgr;	/* General Configuration Register */
 #define MPC85xx_GENCFGR_SDHC_WP_INV	0x20000000
 #else