You mean with the 3.18 vendor kernel? If so, maxcpus=5 prevents the SoC from overheating during early kernel boot, after that there is kernel code to automatically "plug" and "unplug" cores as they are needed. This is a power management technique similar to cpufreq... there is a bit of code what watches the load on the current CPUs (and also the Soc temperature) and adds/removes cores as required. If you run something that consumes cycles you should see new CPUs added in.
For mainline kernel there is (or at least was) very little power management code so maxcpus=5 doesn't just affect early kernel boot... we are limited to five cpus all the time!