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 Behún <marek.behun@nic.cz>

drivers/mtd/nand/raw/mxc_nand_spl.c

@@ -326,7 +326,7 @@ int nand_spl_load_image(uint32_t from, unsigned int size, void *buf)
  * 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);