cacheinfo: Use of_cpu_device_node_get instead cpu_dev->of_node

The of_cpu_device_node_get takes care of fetching the CPU'd device node
either from cached cpu_dev->of_node if cpu_dev is initialised or uses
of_get_cpu_node to parse and fetch node if cpu_dev isn't available yet.

Just use of_cpu_device_node_get instead of getting the cpu device first
and then using cpu_dev->of_node for two reasons:
1. There is no other use of cpu_dev and can be simplified
2. It enabled the use detect_cache_attributes and hence cache_setup_of_node
   much earlier before the CPUs are registered as devices.

Tested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>