ARM: imx: enable suspend for imx6sl

The imx6sl low power mode implementation inherits imx6q/dl one,
and pm-imx6q.c can just work for imx6sl with some minor updates.
Let's enable imx6sl suspend support by reusing pm-imx6q.c and use
cpu_is_imxXX() to handle the those minor differences between imx6sl
and imx6q/dl.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/arch/arm/mach-imx/mxc.h b/arch/arm/mach-imx/mxc.h
index 99e03ea..b08ab3a 100644
--- a/arch/arm/mach-imx/mxc.h
+++ b/arch/arm/mach-imx/mxc.h
@@ -153,6 +153,11 @@
 #endif
 
 #ifndef __ASSEMBLY__
+static inline bool cpu_is_imx6sl(void)
+{
+	return __mxc_cpu_type == MXC_CPU_IMX6SL;
+}
+
 static inline bool cpu_is_imx6dl(void)
 {
 	return __mxc_cpu_type == MXC_CPU_IMX6DL;