microblaze: spl: Do not call mem_malloc_init and use early alloc

This patch has some parts connected together:
- Use _gd in bss section which is automatically cleared
  Location at SPL_MALLOC_END wasn't cleared at all
- Use MALLOC_F_LEN(early alloc) instead of FULL MALLOC
  (mem_malloc_init is not called at all)
- Simplify malloc and stack init.
  At the end of SPL addr is malloc area and below is stack

Signed-off-by: Michal Simek <michal.simek@xilinx.com>