ARM: atmel: sama5d4: move get_cpu_name to device file

As the SAMA5D4 family has SAMA5D41, SAMA5D42, SAMA5D43 and SAMA5D44
members, we need to distinguish them. So, using get_cpu_name to get
the real name of them.

Signed-off-by: Bo Shen <voice.shen@atmel.com>