powerpc/64s: Always disable branch profiling for prom_init.o

Otherwise the build fails because prom_init is calling symbols it's
not allowed to, eg:

  Error: External symbol 'ftrace_likely_update' referenced from prom_init.c
  make[3]: *** [arch/powerpc/kernel/Makefile:197: arch/powerpc/kernel/prom_init_check] Error 1
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191106051129.7626-1-mpe@ellerman.id.au

arch/powerpc/kernel/Makefile

@@ -22,6 +22,7 @@ CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 
 CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
+CFLAGS_prom_init.o += -DDISABLE_BRANCH_PROFILING
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code
@@ -39,7 +40,6 @@ KASAN_SANITIZE_btext.o := n
 ifdef CONFIG_KASAN
 CFLAGS_early_32.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_cputable.o += -DDISABLE_BRANCH_PROFILING
-CFLAGS_prom_init.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_btext.o += -DDISABLE_BRANCH_PROFILING
 endif