mem: Eliminate the Base(Slave|Master)Port classes.

mem: Eliminate the Base(Slave|Master)Port classes.

The Port class has assumed all the duties of the less generic
Base*Port classes, making them unnecessary. Since they don't add
anything but make the code more complex, this change eliminates them.

Change-Id: Ibb9c56def04465f353362595c1f1c5ac5083e5e9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20236
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>