[RFC] SPI

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RFC on SPI gpio implementation. Note two SPI gpio characteristics for the RT6x:

1. SPI signals have multiple options for gpio pin mapping.
2. Up to four chip selects per spi channel

Please take a look at the following lines in src/spi.rs and comment on viability:

• lines 52 - 80, impl<'d, FC: Instance, M: Mode> Spi<'d, FC, M>
• Pin implementations beginning at line 234

Two additional features that may need to be added are:

1. SPI sometimes is implemented with a programmed chip select output, not driven by the hardware peripheral, but requiring software enablement. I expect the SselPin implementations would need to change to account for that.
2. SPI -may- have any number of chip selects, 0 to n. To account for that, the new() apis would have to allow sseln: args to be "optional".

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note how signals and pins may be optionally mapped to multiple locations ...

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This will need additional changes to account for 0 - 4 ssels enabled for an actual instantiation and usage in a firmware build.