mx6: add support of multi-processor command

This allows u-boot to load different OS or Bare Metal application on
different cores of the i.MX6 SoC.
For example: running Android on cpu0 and a RT OS like QNX/FreeRTOS on cpu1.

Signed-off-by: Gabriel Huau <contact@huau-gabriel.fr>
Acked-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/include/asm/arch-mx6/sys_proto.h b/arch/arm/include/asm/arch-mx6/sys_proto.h
index 42d30f5..306d699 100644
--- a/arch/arm/include/asm/arch-mx6/sys_proto.h
+++ b/arch/arm/include/asm/arch-mx6/sys_proto.h
@@ -14,6 +14,7 @@
 #define soc_rev() (get_cpu_rev() & 0xFF)
 #define is_soc_rev(rev)        (soc_rev() - rev)
 
+u32 get_nr_cpus(void);
 u32 get_cpu_rev(void);
 
 /* returns MXC_CPU_ value */