Writing to peripheral register from uboot using 'mw' command fails in TX2

It’s worth following up to this answer to mention that the registers associated with clocking and reset (TRM Chapter 4.6) are not accessible from the CCPLEX. You need to communicate with the BPMP to enable the clock and to deassert reset to whatever device you want to use, be it SPI or I2C. I found a reference implementation for the BPMP communication protocol (called IVC) in the latest U-Boot source for TX2.