dsp: utils: Added macros to shift and convert float/fixed types

[mrcamilletti]

The DSP library does not provide functions to shift and convert fixed representations in Q7, Q15, and Q31 formats to float or double and vice versa. This commit implements those macros using a shift argument to indicate the number of decimal places.

[github-actions]

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[mrcamilletti]

Hi @yperess, @stephanosio. Is there any possibility to review this?
Cheers!

[yperess]

Minor nit on the documentation of the macros. But overall LGTM

[github-actions]

Hi @mrcamilletti!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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