sama5: use correct soc and board names: sama5d3 for soc name, sama5d3xek for board name.

In this patch, we
  1. change the soc file and board directory to correct name.
  2. change the macro name from CONFIG_AT91SAMA5 to CONFIG_SAMA5D3.
  3. change the defconfig name to sama5d3xek_xxx.
sama5: use correct variable names which used in driver.
sama5d3: Use correct chip name for functions that get cpu types.
sama5: use correct board name.
sama5: remove useless macro.