urukul: support per-channel IO_UPDATE

[jordens]

Currently IO_UPDATE targets all four channels. This means that with the auto-clear phase accumulator bit set it will reset all four phase accumulators. To implement independent absolute (and tracking/coherent) phase updates, in addition to the profile register, the configuration register needs to be set twice for each update.

Implement per-channel gating in CPLD gateware (quartiq/urukul#3) and use in the coredevice driver to place the IO_UPDATE pulse in the channel-gated window of the SPI transfer.
The changes in the CPLD gateware and the coredevice driver can be made backwards compatible.

c.f. #1143 for absolute phase mode