ARM: imx6: build headsmp.o only on CONFIG_SMP

With v7_cpu_resume() being moved out of headsmp.S, all the remaining
code in the file is only needed by CONFIG_SMP build.  So we can control
the build of headsmp.o with only obj-$(CONFIG_SMP) now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/mach-imx/headsmp.S

@@ -15,7 +15,6 @@
 
 	.section ".text.head", "ax"
 
-#ifdef CONFIG_SMP
 diag_reg_offset:
 	.word	g_diag_reg - .
 
@@ -32,4 +31,3 @@ ENTRY(v7_secondary_startup)
 	set_diag_reg
 	b	secondary_startup
 ENDPROC(v7_secondary_startup)
-#endif