arm: imx: do not include smp_wfe_imx6.S for i.MX6SX

smp_wfe_imx6 is only used for i.MX6Q. Including it in i.MX6SX-only
configurations causes two build issues:

- The symbols it defines conflict with fallback definitions in common.c
- It scu_power_mode, which is undefined without HAVE_ARM_SCU

Signed-off-by: Matthias Schiffer <matthias.schiffer@tq-group.com>

arch/arm/mach-imx/Makefile

@@ -91,7 +91,7 @@ obj-$(CONFIG_SOC_IMX6Q) += mach-imx6q.o ddr3_freq_imx6.o smp_wfe_imx6.o \
 				lpddr2_freq_imx6q.o
 obj-$(CONFIG_SOC_IMX6SL) += mach-imx6sl.o lpddr2_freq_imx6.o
 obj-$(CONFIG_SOC_IMX6SLL) += mach-imx6sl.o lpddr2_freq_imx6sll.o
-obj-$(CONFIG_SOC_IMX6SX) += mach-imx6sx.o ddr3_freq_imx6sx.o smp_wfe_imx6.o lpddr2_freq_imx6sx.o
+obj-$(CONFIG_SOC_IMX6SX) += mach-imx6sx.o ddr3_freq_imx6sx.o lpddr2_freq_imx6sx.o
 obj-$(CONFIG_SOC_IMX6UL) += mach-imx6ul.o
 obj-$(CONFIG_SOC_IMX7D_CA7) += mach-imx7d.o pm-imx7.o ddr3_freq_imx7d.o smp_wfe.o \
 			   lpddr3_freq_imx.o suspend-imx7.o