ARM: i.MX: provide declaration for board_spi_cs_gpio

Provide a public declaration of the board_spi_cs_gpio()
callback for i.MX SPI chip selects to prevent the warning
"Should it be static?" when compiling with "make C=1".

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
diff --git a/board/freescale/mx6slevk/mx6slevk.c b/board/freescale/mx6slevk/mx6slevk.c
index a0832f4..a500133 100644
--- a/board/freescale/mx6slevk/mx6slevk.c
+++ b/board/freescale/mx6slevk/mx6slevk.c
@@ -13,6 +13,7 @@
 #include <asm/arch/sys_proto.h>
 #include <asm/gpio.h>
 #include <asm/imx-common/iomux-v3.h>
+#include <asm/imx-common/spi.h>
 #include <asm/io.h>
 #include <linux/sizes.h>
 #include <common.h>