ARM: i.MX6: include prototype for get_board_rev()

Include <asm/bootm.h> to see the prototype for get_board_rev()
and prevent warning "Should it be static?" with "make C=1".

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
diff --git a/arch/arm/cpu/armv7/mx6/soc.c b/arch/arm/cpu/armv7/mx6/soc.c
index 6352422..6dc2600 100644
--- a/arch/arm/cpu/armv7/mx6/soc.c
+++ b/arch/arm/cpu/armv7/mx6/soc.c
@@ -20,6 +20,7 @@
 #include <stdbool.h>
 #include <asm/arch/mxc_hdmi.h>
 #include <asm/arch/crm_regs.h>
+#include <asm/bootm.h>
 
 enum ldo_reg {
 	LDO_ARM,