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>

arch/arm/mach-at91/Makefile

@@ -87,7 +87,7 @@ obj-$(CONFIG_MACH_SNAPPER_9260)	+= board-snapper9260.o
 obj-$(CONFIG_MACH_AT91SAM9M10G45EK) += board-sam9m10g45ek.o
 
 # AT91SAM board with device-tree
-obj-$(CONFIG_MACH_AT91RM9200_DT) += board-rm9200-dt.o
+obj-$(CONFIG_MACH_AT91RM9200_DT) += board-dt-rm9200.o
 obj-$(CONFIG_MACH_AT91SAM9_DT) += board-dt-sam9.o
 
 # AT91X40 board-specific support