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/embest/mx6boards/mx6boards.c b/board/embest/mx6boards/mx6boards.c
index a725f15..02fb3fa 100644
--- a/board/embest/mx6boards/mx6boards.c
+++ b/board/embest/mx6boards/mx6boards.c
@@ -23,6 +23,7 @@
 #include <asm/imx-common/iomux-v3.h>
 #include <asm/imx-common/boot_mode.h>
 #include <asm/imx-common/mxc_i2c.h>
+#include <asm/imx-common/spi.h>
 #include <asm/imx-common/video.h>
 #include <i2c.h>
 #include <mmc.h>