Change in coreboot[master]: soc/amd/common/block: Add spi_hw mutex

[Raul Rangel (Code Review)]

Attention is currently required from: Jason Glenesk, Marshall Dawson, Julius Werner, Rob Barnes, Karthik Ramasubramanian, Felix Held.
Raul Rangel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/58926 )

Change subject: soc/amd/common/block: Add spi_hw mutex
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Patch Set 2:

(1 comment)

File src/soc/amd/common/block/spi/fch_spi_ctrl.c:

https://review.coreboot.org/c/coreboot/+/58926/comment/c8f4050c_c76c56ee 
PS2, Line 147: thread_mutex_lock
So one unfortunate thing is that we can only transfer ~60 bytes pre transaction, so we lock and unlock the mutex a bunch of times when writing to the SPI ROM. This does cause the printk in thread_mutex_lock to spam a bunch during `elog init`.

There is a .claim_bus and .free_bus callback, but they get called before and after xfer_vectors, so it's not different. Ideally a .claim_bus would get called once, then all the transfers, then release, but that is a pretty big change...