mpc83xx: Introduce ARCH_MPC837X

Replace CONFIG_MPC837x with a proper CONFIG_ARCH_MPC837X Kconfig option.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/arch/powerpc/include/asm/immap_83xx.h b/arch/powerpc/include/asm/immap_83xx.h
index f6721e7..30bbd56 100644
--- a/arch/powerpc/include/asm/immap_83xx.h
+++ b/arch/powerpc/include/asm/immap_83xx.h
@@ -759,7 +759,7 @@
 	u8			res12[0x1CF00];
 } immap_t;
 
-#elif defined(CONFIG_MPC837x)
+#elif defined(CONFIG_ARCH_MPC837X)
 typedef struct immap {
 	sysconf83xx_t		sysconf;	/* System configuration */
 	wdt83xx_t		wdt;		/* Watch Dog Timer (WDT) Registers */