machine/dp8390.cpp: convert I/O map to new standards

[angelosa]

DP8390 a.k.a. NE2000 network controller chip needs modernization of the internal I/O map, being essentially a 4 paged view selection from m_regs.cr in the first 4 bits of address space. This is a necessity for RTL8029 being a minor update of RTL8019.