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>

arch/microblaze/cpu/start.S

@@ -162,7 +162,11 @@ clear_bss:
 #endif
 	brai	board_init_f
 #else
-	addi	r31, r0, CONFIG_SYS_SPL_MALLOC_END
+	addi	r31, r0, _gd
+#if defined(CONFIG_SYS_MALLOC_F_LEN)
+	addi	r6, r0, CONFIG_SPL_STACK_ADDR
+	swi	r6, r31, GD_MALLOC_BASE
+#endif
 	brai	board_init_r
 #endif
 1:	bri	1b