spl: Add prototype to function spl_board_boot_device()

Add prototype to function spl_board_boot_device to fix the following
sparse warning:

board/freescale/imx8mm_evk/spl.c:26:5: warning: no previous prototype
for ‘spl_board_boot_device’ [-Wmissing-prototypes]

Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>
diff --git a/include/spl.h b/include/spl.h
index cd4669f..6087cd7 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -269,6 +269,7 @@
 void spl_board_prepare_for_linux(void);
 void spl_board_prepare_for_boot(void);
 int spl_board_ubi_load_image(u32 boot_device);
+int spl_board_boot_device(u32 boot_device);
 
 /**
  * jump_to_image_linux() - Jump to a Linux kernel from SPL