powerpc/boot: Fix build failure in 32-bit boot wrapper

OPAL is not callable from 32-bit mode and the assembly code for it
may not even build (depending on how binutils was configured).

References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=powerpcspe&ver=4.8.7-1&stamp=1479203712
Fixes: 656ad58ef19e ("powerpc/boot: Add OPAL console to epapr wrappers")
Cc: [e-mail] # v4.8+
Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/boot/Makefile

···
 		ns16550.c serial.c simple_alloc.c div64.S util.S \
 		elf_util.c $(zlib-y) devtree.c stdlib.c \
 		oflib.c ofconsole.c cuboot.c mpsc.c cpm-serial.c \
-		uartlite.c mpc52xx-psc.c opal.c opal-calls.S
+		uartlite.c mpc52xx-psc.c opal.c
+src-wlib-$(CONFIG_PPC64_BOOT_WRAPPER) +=  opal-calls.S
 src-wlib-$(CONFIG_40x) += 4xx.c planetcore.c
 src-wlib-$(CONFIG_44x) += 4xx.c ebony.c bamboo.c
 src-wlib-$(CONFIG_8xx) += mpc8xx.c planetcore.c fsl-soc.c

arch/powerpc/boot/opal.c

···
 #include <libfdt.h>
 #include "../include/asm/opal-api.h"
 
-#ifdef __powerpc64__
+#ifdef CONFIG_PPC64_BOOT_WRAPPER
 
 /* Global OPAL struct used by opal-call.S */
 struct opal {