arch_topology: Add support for parsing sockets in /cpu-map

Finally let us add support for socket nodes in /cpu-map in the device
tree. Since this may not be present in all the old platforms and even
most of the existing platforms, we need to assume absence of the socket
node indicates that it is a single socket system and handle appropriately.

Also it is likely that most single socket systems skip to as the node
since it is optional.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>