decodetree: Allow +- to begin a number initializing a field

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

scripts/decodetree.py

@@ -589,7 +589,7 @@ def parse_generic(lineno, is_format, name, toks):
             continue
 
         # 'Foo=number' sets an argument field to a constant value
-        if re_fullmatch(re_ident + '=[0-9]+', t):
+        if re_fullmatch(re_ident + '=[+-]?[0-9]+', t):
             (fname, value) = t.split('=')
             value = int(value)
             flds = add_field(lineno, flds, fname, ConstField(value))