ARM: at91: rename board-dt to more specific name board-dt-sam9

We will produce a board-dt file per SoC core type. That will ease code
readability and will prevent from including superfluous code for supporting
machines that will never be compiled together (particularly the ARM9 and C-A5
upcoming SoCs).

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
[nicolas.ferre@atmel.com: modify commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>