By Taiga Uranaka and Makiko Yamazaki TOKYO (Reuters) – Japanese trust banks are preparing to sue Toshiba Corp over its 2015 accounting scandal, a fresh headache for the conglomerate as it scrambles to offset a separate imminent multi-billion dollar writedown. Chairman Shigenori Shiga is ready to step down to take responsibility for the upcoming charge – estimated at around $6 billion, local media have also reported. The announcements on Friday failed to clear up much of the uncertainty surrounding Toshiba and its shares lost 4 percent in Monday morning trade.