With PBL FAT support implemented, provide an sama5d2_sdhci_start_image
helper that can be called from the PBL to chainload a barebox.bin file
from the first FAT partition.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>