interfacez_io.vhd address match is using a cryptic address match condition

[sky-ruimartins]

ZXInterfaceZ/fpga/rtl/common/interfacez_io.vhd

Line 173 in 601a4b2

if adr_i(0)='1' and adr_i(1)='1' and adr_i(5)='1' and adr_i(7)='0' then

Instead of using:
if adr_i(0)='1' and adr_i(1)='1' and adr_i(5)='1' and adr_i(7)='0' then

define constantes for a MASK and MATCH value, so that these magic numbers disappear and it becomes easier to read.

[alvieboy]

Will also restructure the packages so that we have only a single port listing instead of two like we have right now.
The matches will also use a different approach, by using "x" placeholders instead of value/mask pairs. This makes them easier to read and less error prone (at least one of the masks seems to be currently incorrect)