The constants EXT_CFG_DATA and EXT_CFG_INDEX vary by SOC. One of the
map_bus methods used these constants, the other used different constants.
Fortunately there was no problem because the SoCs that used the latter
map_bus method all had the same register constants.
Remove the redundant constants and adjust the code to use them. In
addition, update EXT_CFG_DATA to use the 4k-page based config space access
system, which is what the second map_bus method was already using.
Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>