ARM: fix LTO for apf27

When apf27_defconfig is built with LTO, linking complains about
undefined reference to `nand_boot`. This is because it is referenced
from inline assembly. Make it visible.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/drivers/mtd/nand/raw/mxc_nand_spl.c b/drivers/mtd/nand/raw/mxc_nand_spl.c
index e1e5425..2f054b6 100644
--- a/drivers/mtd/nand/raw/mxc_nand_spl.c
+++ b/drivers/mtd/nand/raw/mxc_nand_spl.c
@@ -326,7 +326,7 @@
  * configured and available since this code loads the main U-Boot image
  * from NAND into SDRAM and starts it from there.
  */
-void nand_boot(void)
+__used void nand_boot(void)
 {
 	__attribute__((noreturn)) void (*uboot)(void);