The easy way

Download the prebuilt module package
Tick the box in the Ghidra module manager, restart.

Setting up the dev environment

Now organized as a ghidra extension.
Launch Ghidra from Eclipse (install GhidraDev plugin!)
In Ghidra, run sleigh reload script in script manager. It will call sleigh compiler automatically.

Manually compile/update sleigh files

There's both sleigh.exe and sleigh.bat.

# actual SLEIGH compiler
C:\ProgramData\chocolatey\lib\ghidra\ghidra_9.1.1_PUBLIC\support\sleigh.bat -e dsp56k
# does syntax checking, but no output(?)
C:\ProgramData\chocolatey\lib\ghidra\ghidra_9.1.1_PUBLIC\Ghidra\Features\Decompiler\os\win64\sleigh.exe -e dsp56k

When the compiler exits successfully, it generates dsp56k.sla Copy the whole dsp56k_ghidra folder to Ghidra_installation/Ghidra/Processors/

Note, this conflicts with module installation.

Limitations and known issues

A lot of instructions generate wrong CCR flags. The decompiler may output nonsense.
You'll see a lot of shifting, due to the layout of the register file and the instruction semantics.
Nested DO loop is supported, up to 3 layers. But it doesn't work everytime. "Repair flow" seems to help Ghidra to sort out the contextual state and correct the state flow.
Not 100% instruction coverage. See #1 .
Address Modifiers are currently ignored, meaning only linear addressing is supported at the moment.