bootm: Move do_bootm_states() comment to header file

This is an exported function, so move the function comment to the
bootm.h header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>