hdl.mem: use 1 as reset value for ReadPort.en.

hdl.mem: use 1 as reset value for ReadPort.en.

This is necessary for consistency, since for transparent read ports,
we currently do not support .en at all (it is fixed at 1) due to
YosysHQ/yosys#760. Before this commit, changing port transparency
would require adding or removing an assignment to .en, which is
confusing and error-prone.

Also, most read ports are always enabled, so this behavior is also
convenient.