arm: socfpga: fix SPL on gen5 after moving to DM serial

arm: socfpga: fix SPL on gen5 after moving to DM serial

There were NULL pointers dereferenced because DM was used
too early without correct initialization:
- malloc_simple returned NULL when called from preloader_console_init()
  because gd->malloc_limit was 0
- uclass_add dereferenced gd->uclass_root members which were NULL because
  dm_init (or one of its relatives) has not been called.

All this is fixed by calling spl_early_init before calling
preloader_console_init.

This fixes commit 31af9164470e ("ARM: socfpga: Convert to DM serial")

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