ARM: da850evm: Split MTDPARTS into SPL and u-boot

The MTDPARTS currently lists just u-boot.ais as 512k in size.
This works when loading the ais file via serial port, but if one
wanted to update just the u-boot portion, it's not really possible.
This patch splits the MTDPARTS into a 32k SPL partiion and a 480k
u-boot partition which allows u-boot.img to be burned to the u-boot
partition.  The remaining partitions are left with the same sizes
and offsets to not break backwards compatibility.

Signed-off-by: Adam Ford <aford173@gmail.com>