arch: Wrap a docstring in isa_parser.py.

This brings the ISA parser in line with the style guide. Note that the
docstring needs to be a single string literal for python to consider it
a docstring, and the parser itself needs each line of the docstring to
be a rule in its CFG. We can accomplish both by taking advantage of the
fact that two directly adjacent quoted strings are treated as a single
string literal by python, and by escaping the newline so that they're
actually considered adjacent.

Change-Id: I7f4d252998877808425aafb0159600ba4c3bf9ad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35276
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/arch/isa_parser.py b/src/arch/isa_parser.py
index d69bea2..4f69a3f 100755
--- a/src/arch/isa_parser.py
+++ b/src/arch/isa_parser.py
@@ -2082,7 +2082,8 @@
     # 'def [signed] bitfield <ID> [<first>:<last>]'
     # This generates a preprocessor macro in the output file.
     def p_def_bitfield_0(self, t):
-        'def_bitfield : DEF opt_signed BITFIELD ID LESS INTLIT COLON INTLIT GREATER SEMI'
+        'def_bitfield : DEF opt_signed ' \
+                'BITFIELD ID LESS INTLIT COLON INTLIT GREATER SEMI'
         expr = 'bits(machInst, %2d, %2d)' % (t[6], t[8])
         if (t[2] == 'signed'):
             expr = 'sext<%d>(%s)' % (t[6] - t[8] + 1, expr)