spl: Rework and tighten some dependencies

- In a few places, add missing "depends on" that can be implied from the
  option name (i.e. SPL_DM_xxx depends on SPL_DM).
- Make less use of "if SPL_xxx ... endif" clauses as most of the time
  this reads better as depends on.  In the case of UBI however, move it
  all to a sub-menu.
- Rework SPL_NO_CPU_SUPPORT as it's very specific to the
  non-SPL_FRAMEWORK implementation used on those platforms, and a
  tangent to how CONFIG_SPL_START_S_PATH was used.

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