Freescale does an excellent job of putting all of their stuff up on Github. The iMX6 family is a joy to work with thanks to their efforts.
They also go above and beyond with documentation. No NDA/MDA needed. Tens of thousands of pages of freely available technical reference including the entire TRMs for the SoCs:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?co...
For instance, the vpu kernel driver (GPL license) is now open: https://github.com/omegamoon/rockchip-rk30xx-mk808/blob/mast...
And the libstagefright (includes libvpu) as Apache license: https://github.com/HermanChen/Rockchip_4.2_release_libstagef...
The Mali GPU driver is still stuck in proprietary limbo though, AFAIK. Rockchip can't release the code without ARM's permission, and ARM has only released partial support as of now. They claim that it's working, though I am still trying to get a complete X11/Wayland driver running myself (mostly due to all the android dependencies which I don't need under pure Linux)
http://community.arm.com/message/21331 http://malideveloper.arm.com/develop-for-mali/drivers/open-s...
So things are getting better, though there is still some wide gaping holes in development.
That depends on the end manufacturer. There are lots of Mediatek source code dumps on Github, for example. The problem is that the chipset manufacturers don't actively tell their customers what they are obliged to do under the license - and so they assume that the chipset vendor has given the code and docs under "gongkai" terms.