Make DmaDescriptor methods public

[Dominaezzz]

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Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
• I have used cargo xtask fmt-packages command to ensure that all changed code is formatted correctly.
• My changes were added to the CHANGELOG.md in the proper section.
• I have added necessary changes to user code to the Migration Guide.
• My changes are in accordance to the esp-rs API guidelines

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Pull Request Details 📖

Description

This makes all the private methods for manipulating the DmaDescriptors public.
The motivation for this is to allow users provide custom implementations of DmaTxBuffer and DmaRxBuffer.

Testing

It builds

[bugadani]

Besides the nits I have to pick w.r.t. the documentation, I think this is a valuable change. I would like to see a community crate taking advantage of this and alloc.

[bugadani]

Thanks!