spl: Add functions for next and previous phase

It is useful to be able to figure out which phase we are loading next and
which phase we came from. Add some functions to handle this as well as
returning the name of a phase. This allows messages like "Booting to x"
where x is the next phase.

At present, TPL says 'Jumping to U-Boot' at the end, when in fact it is
jumping to SPL. This is confusing, so use the new functions to correct
this.

Tests for this will come with an upcoming minor SPL test refactor.

Signed-off-by: Simon Glass <sjg@chromium.org>