[nfc] Use EBNF for ABI probe macros, filename

Minor cleanup to the ABI section on probes and port lowering ABIs.  Use
EBNF code blocks to make these easier to read.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

abi.md

@@ -98,7 +98,18 @@ module Top(
 
 Ports of aggregate type shall be scalarized according to the "Aggregate Type Lowering" description in the FIRRTL spec.
 
-Ports of ref type on public modules shall, for each public module, be lowered to a Verilog macro of the form `` `define ref_<module name>_<portname> <internal path from module> `` in a file with name `ref_<module name>.sv`.
+Ports of ref type on public modules shall, for each public module, be lowered to a Verilog macro with the following format where `module` is the name of the public module, `portname` is the name of the port, and `internalpath` is the hierarchical path name:
+
+``` ebnf
+macro = "`define " , "ref_" , module , "_" , portname , " ", internalpath
+```
+
+All macros for a public module will be put in a file with name:
+
+``` ebnf
+filename = "ref_" , module , ".sv"
+```
+
 References to aggregates will be lowered to a series of references to ground types.
 This ABI does not specify whether the original aggregate referent is scalarized or not.
 

revision-history.yaml

@@ -17,6 +17,7 @@ revisionHistory:
       - Add ABI for public modules and filelist output.
       - Changed ABI for group and ref generated files.
         These now use the public module and not the circuit.
+      - Use EBNF to describe probe port macros and filename.
   # Information about the old versions.  This should be static.
   oldVersions:
     - version: 3.2.0