ARM: at91: remove at91rm9200 legacy board support

Second part of at91rm9200 legacy !DT removal. This is the core !DT support
removal for this Atmel SoC.
Note that from now on, the Kconfig.non_dt file and its specialized options are
completely removed.
Use the Device Tree for running this board with newer kernels.

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