Boot media: save boot media to the backup register

Give the boot media information provided by the romcode through R4 to next
level bootloader.

This information is stored in the second GPBR for sama5d3 family.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>