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>