arm: socfpga: fix comment about SPL memory layout

The comment about SPL memory layout for socfpga gen5 is outdated: the
initial malloc memory is now at the end of the SRAM, gd is below it
(see board_init_f_alloc_reserve).

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