add at91rm9200-ek board support

add at91rm9200-ek board support

tested with jlink sam-ice v5 while loading barebox

(gdb) load
Loading section .text, size 0x2f190 lma 0x21f00000
Loading section .rodata, size 0x931c lma 0x21f2f190
Loading section .data, size 0x29e8 lma 0x21f384ac
Loading section .barebox_cmd, size 0x78c lma 0x21f3ae94
Loading section .barebox_initcalls, size 0x80 lma 0x21f3b620
Start address 0x21f00000, load size 243360
Transfer rate: 26 KB/sec, 13520 bytes/write.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Patrice Vilchez <patrice.vilchez@atmel.com>