ARM: at91: sama5d3xek: use a $dtb_name to load dtb

Since sama5d3xek boards has different type of dtb blobs, so we need to detect
the cpu type in runtime.

So we add a new variable $dtb_name. if $dtb_name is not defined, we just use
	at91-${board_name}.dtb

as the $dtb_name. Otherwise, we will just load the dtb with
	$dtb_name.

For sama5d3xek, we will detect cpu type and make up $dtb_name in
runtime.

Signed-off-by: Josh Wu <josh.wu@atmel.com>