Atomic: Remove the physmem_port and access memory directly

This patch removes the physmem_port from the Atomic CPU and instead
uses the system pointer to access the physmem when using the fastmem
option. The system already keeps track of the physmem and the valid
memory address ranges, and with this patch we merely make use of that
existing functionality. As a result of this change, the overloaded
getMasterPort in the Atomic CPU can be removed, thus unifying the CPUs.

util/regress all passing (disregarding t1000 and eio), 
also running se.py and fs.py with --fastmem