fit: Rework SPL_LOAD_FIT_ADDRESS slightly

fit: Rework SPL_LOAD_FIT_ADDRESS slightly

Options which deal with memory locations and have a default value of 0x0
are dangerous, as that is often not a valid memory location. Rework
SPL_LOAD_FIT_ADDRESS as follows:
- Add SPL_HAS_LOAD_FIT_ADDRESS to guard prompting the question as the
  case of loading a FIT image does not strictly require setting an
  address and allows for a malloc()'d area to be used.
- For SPL_RAM_SUPPORT, select the new guard symbol if SPL_LOAD_FIT is
  enabled because in that case an address must be provided.
- Update defconfigs for these new changes. Largely this means some
  defconfigs need to enable SPL_HAS_LOAD_FIT_ADDRESS to maintain their
  current status. In the case of sandbox, we also need to set
  SPL_LOAD_FIT_ADDRESS to 0x0.

Signed-off-by: Tom Rini <trini@konsulko.com>