imx: add i.MX 6SLL CPU type

Add i.MX6SLL cpu type.
MXC_CPU_MX6D is not a real value in chip, so change it to 0x6A.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/include/asm/imx-common/sys_proto.h b/arch/arm/include/asm/imx-common/sys_proto.h
index 005435a..539d34b 100644
--- a/arch/arm/include/asm/imx-common/sys_proto.h
+++ b/arch/arm/include/asm/imx-common/sys_proto.h
@@ -36,6 +36,7 @@
 #define is_mx6solo() (is_cpu_type(MXC_CPU_MX6SOLO))
 #define is_mx6ul() (is_cpu_type(MXC_CPU_MX6UL))
 #define is_mx6ull() (is_cpu_type(MXC_CPU_MX6ULL))
+#define is_mx6sll() (is_cpu_type(MXC_CPU_MX6SLL))
 
 u32 get_nr_cpus(void);
 u32 get_cpu_rev(void);