bank: add RE signal for registers made of fields

migen/bank/csrgen.py

@@ -34,6 +34,7 @@ class Bank:
 						bwra.append(field.storage.eq(self.interface.dat_w[offset:offset+field.size]))
 					offset += field.size
 				if len(bwra) > 1:
+					bwra.append(reg.re.eq(1))
 					bwcases.append(bwra)
 			else:
 				raise TypeError

migen/bank/description.py

@@ -27,9 +27,13 @@ class Field:
 				self.we = Signal()
 
 class RegisterFields:
-	def __init__(self, name, fields):
+	def __init__(self, name, fields, re=None):
 		self.name = name
 		self.fields = fields
+		if re is None:
+			self.re = Signal()
+		else:
+			self.re = re
 
 class RegisterField(RegisterFields):
 	def __init__(self, name, size=1, access_bus=READ_WRITE, access_dev=READ_ONLY, reset=0):
@@ -71,7 +75,7 @@ def expand_description(description, busword):
 				else:
 					f.append(field)
 			if f:
-				d.append(RegisterFields(reg.name, f))
+				d.append(RegisterFields(reg.name, f, reg.re))
 		else:
 			raise TypeError
 	return d