arm: socfpga: put initial U-Boot stack into DDR

If SPL post-reloc stage puts the stack into DDR, U-Boot should be able to
do that, too.

The reason to do so is that this way, U-Boot initial stack can be larger
than SPL initial stack. In situations where we want to save the SPL
in SRAM for next boot without reloading, this prevents overwriting the
SPL DTB in SRAM if U-Boot stack usage gets too high.

To achieve this, the malloc definition for a10 is moved up and sligthly
changed to ensure CONFIG_SYS_INIT_RAM_SIZE is the remaining available size.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>