ARM: at91: remove at91rm9200 legacy boards files

Remove old board files that use at91rm9200 Atmel SoC. The
device tree is mature on this SoCs. It must be used now.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>