Change in flashrom[master]: buspirate_spi: Add support for variable serial speeds

[Shawn Anastasio (Code Review)]

Shawn Anastasio has posted comments on this change. ( https://review.coreboot.org/23057 )

Change subject: buspirate_spi: Add support for variable serial speeds
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Patch Set 3:

My reasoning is that the Bus Pirate itself offers options for different baud rates, including many of the values included in the table. I just thought it best to expose as much of the Bus Pirate's feature set as possible and let the user choose between them. The extra values in my table are intermediate values that the Bus Pirate's menu doesn't offer by default but are still slower than the max of 2M.  Also, the table omits values too slow to be usable (<9600 baud) for this purpose.