Hi,
Thank you for your interest in NXP Semiconductor products,
DSPs are used through their firmware, please refer to https://www.nxp.com/docs/en/user-guide/IMX_DSP_USERS_GUIDE.pdf.
I found the chapter about NatureDSP Library Support, is the library that matches the common operations of a DSP.
Used signal processing functions: FFT, FIR, vector, matrix, and common mathematics.
For UART expansion, you could either look for multiplexer/bridges or simulate a UART from GPIO, that development would be up to you.
Regards