generally yes, they are documented and pins, registers are accessible. there might be even "programming guidelines/notes" helping you to get the idea what to do with all that thing. xD if you are lucky. i2c/smbus controllers are documented as well. if the programming manual isn't crappy, then it's possible. it happens that the vendor throws some joke looking piece that it calls "manual", then you have to research their BSP code in addition, they all use linux/uboot, so they are obligated to publish it. it's a good idea to look at the documentation quality for the SoC in question before, rather than hastily buying the board just because it's so cute and numbers are so intriguing. like for example in the case of mentioned EsspressoBin board that is as good as Marvel is bad in providing needed resources.
on the example of setting the DDRC/DDR PHY, which i've done, the documentation was desperately insufficient, basically i learnt from uboot sources, and they in turn, got it from BSP code. but ddr is a special thing often made that "secret". for example MSC controllers (that deal with eMMC/SD/SDIO, the current task) are much better described. lcd stuff looks the same - loads of tables, register definitions, diagrams etc.
in short here the problem is not the same as with proprietary GPUs, rather the manual quality.
