When a previous phase of U-Boot has run we need to adjust the init of
subsequent states to avoid messing up the CPU state.

Add a new version of the start logic for SPL, when it boots from TPL
(start_from tpl.c) and a new version for U-Boot when it boots from SPL.

Series-changes: 2
- Add xorl to TPL code also
- Update comments in start_from_tpl to correctly explain SPL state

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