Add FlexIO registers to Resources object

[Finomnis]

All three FlexIO drivers are connected to pins that are available on the Teensy4.0.

Examples:

• Teensy 4.0 Pin 5 - EMC_08 - Flexio1, Pin 8
• Teensy 4.0 Pin 6 - B0_10 - Flexio2, Pin 10
• Teensy 4.0 Pin 14 - AD_B1_02 - Flexio3, Pin 2

This change is related to https://github.com/imxrt-rs/imxrt-iomuxc/pull/37.

A driver in imxrt-hal does not exist yet and most likely never will, because FlexIO is just simply too generic. So most likely there will be usecase-specific drivers in separate crates.

Like my usecase; I am in the process of writing a ws2812-flexio driver crate for Neopixel LED Strips.

[mciantyre]

Now that we have clock gates available in imxrt-hal, should we ensure that the FlexIO clock gates are on? This would be consistent with the board's clock policy.

I agree that this peripheral is flexible, so I'm indifferent about making board choose the root clocks and FlexIO input clock frequencies. If we want to skip this, we could document an exception for the FlexIO peripheral. Otherwise, we could choose default clocks and frequencies. It's OK to use imxrt-ral for clock changes until there's corresponding APIs in imxrt-hal.

https://github.com/mciantyre/teensy4-rs/blob/e6eb36d5e73cb018209a9836bd3da32a6c4dd4ef/src/board.rs#L54-L62

[Finomnis]

Now that we have clock gates available in imxrt-hal, should we ensure that the FlexIO clock gates are on? This would be consistent with the board's clock policy.

That would make sense. FlexIO does have default clock frequencies which are specified in the documentation. (see the black dot next to the muxes)

[Finomnis]

@mciantyre Done :)

[Finomnis]

@mciantyre Any news on this? :)

[Finomnis]

@mciantyre Great! Thanks a lot for your collaboration. I highly appreciate the work you do for this community.