ARM: at91: renamme rm9200 dt file

Rename the board-rm9200-dt.c file so that we follow the pattern for
Device Tree board files: board-dt-<name of SoC sharing same core>.c

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>