sbxxx: Cleanup.

 - Move programmer definition to the top.
 - Rewrite array accesses to use indices instead of using pointer arithmetic.
 - Move length check and opcode extraction to a function.
 - Move IMC parameter handling into existing IMC handling function.
 - Split comparing and resetting the FIFO pointer.
sbxxx: Add support for new AMD SPI controller.

This patch adds support for the "SPI 100" SPI engine in Yangtze FCHs
(found in Kabini and Temash).

Tested reading/writing on ASRock IMB-A180.

Signed-off-by: Wei Hu <wei@aristanetworks.com>
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
sbxxx: Add spispeed parameter.

Allow to set the SPI clock frequency on AMD chipsets with a programmer
parameter. If the parameter is given (and matches a possible value), the
SPI clock is set temporarily. Both registers are restored on programmer
shutdown.

Example: ./flashrom -p internal:spispeed="33 MHz" -V

Possible values for spispeed are "16.5 MHz", "22 MHz", "33 MHz", "66 MHz",
"100 MHZ" and "800 kHz" depending on the chipset generation.
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