arm: socfpga: spl_gen5: clean up malloc_base assignment

arm: socfpga: spl_gen5: clean up malloc_base assignment

In spl_gen5's board_init_f(), gd->malloc_base is manually assigned
at the end of the function to point to sdram.  This code is outdated
as by now, the heap is switched to sdram by the common function
spl_relocate_stack_gd() if the appropriate defines are set.

As it was, the value assigned manually was directly overwritten by
this common code, so remove the manual assignment.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>