ast2600: Configure u-boot-with-spl.bin target

For the u-boot-with-spl.bin target to be useful for the AST2600, set the
maximum SPL size which also sets the padding length.

The normal way of loading u-boot is as a FIT, so configure u-boot.img as
the SPL playload.

With this the following simple steps can be used to build and boot a
system:

  make u-boot-with-spl.bin
  truncate -s 64M u-boot-with-spl.bin
  qemu-system-arm -nographic -M ast2600-evb \
    -drive file=u-boot-with-spl.bin,if=mtd,format=raw

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>