Got Linux booting on my rx3715. Problem was in wrong serial port number. I thought that serial port brought to external connector is ttySAC0, but it's ttySAC1 on rx3715. Just changed some values in kernel config - and voila - it boots. Will try to get touch screen, buttons, udc and ASIC3 working a bit later.
Btw, now I have serial cable for iPAQs and JTAG adapter (Olimex). So I hope debugging will be easier now. :)