Freescale and TI are pretty solid in this area. The i.MX6 TRM is a freely available 7000+ page PDF.
AllWinner and RockChip? Not counting on it. I went through an ordeal with Ingenic (nee ChinaChip) where it took three levels of pleading with reps to get a badly-translated 50-page document.
IMO there's a difference between true eval/dev boards and the (for lack of a better word) devices that are following along the lines of the RPi.
Dev boards used to be large expensive things that did require NDAs and a healthy business relationship with a chipmaker and local FAE. Could you afford the $4K for an OMAP1 eval board? I couldn't.
I'm glad to see some of that stuff come back down to earth, especially now that the big players are cooperating with more-open projects like Beagle/Panda and SABRE. And they're at least following it up with a more open attitude when it comes to documentation, drivers/kernel branches, and online support.
You can never get a complete TRM for high performance ARM SoCs because they all license some IP for which they can't publish the documentation, usually for 3D accelerators and video decoders. You can find leaked Allwinner and Rockchip TRMs, at least for Allwinner A20 (the one used by Cubieboard for example) and RK3066 (which is quite similar to RK3188), so it's a compromise I've accepted in a few cases.