[spi_device] Document update needed for flash/passthrough mode

[weicaiyang]

EDIT: make this a checklist to include other document update requests.

• As @eunchan mentioned, we support {polarity, phase} to be 'b00 or 'b11 for flash/passthrough mode. Please document it in the spec.

• since [SPI_DEVICE|SPI_FLASH] Added a write-protection on en4b/ex4b to avoid… #15326 is merged, perhaps also document the minimum inactive time between transactions (2 tpm, 2 flash, 1 tpm and 1 flash).

• document that SW can only update addr_4b at the beginning before host sends any commands, due to [spi_device] SW fails to update addr_4b_en in the middle of sim #15543

• document mem parity in countermeasure section

[weicaiyang]

BTW, since #15326 is merged, perhaps also document the minimum inactive time between transactions (2 tpm, 2 flash, 1 tpm and 1 flash).

cc: @jeoongp

[msfschaffner]

Since this is mostly documentation cleanup in the spec, I will mark this as a D3 / M3 item.

[weicaiyang]

Since this is mostly documentation cleanup in the spec, I will mark this as a D3 / M3 item.

SG, thanks @msfschaffner

[eunchan]

@jeoongp could you please address the second item? You most likely knows better than I am onto the logic.

[andreaskurth]

Triaged for spi_device. Assigning to M2.5 with Priority:P3 Priority: low because this is 'only' about documentation.

[hcallahan-lowrisc]

estimate 4
remaining 2023-03-22 4