WIP "G1-ATA" IDE HDD support. Not enabled by default.

Demonstrated on NetBSD booth at Open Source Conference 2016 Tokyo Fall
and Kansai Open Source 2016:
http://mail-index.netbsd.org/netbsd-advocacy/2016/11/09/msg000721.html
http://mail-index.netbsd.org/netbsd-advocacy/2016/11/16/msg000722.html

See my port-dreamcast@ post for more details:
http://mail-index.netbsd.org/port-dreamcast/2016/12/18/msg000295.html