ARM: at91: switch configuration option to SOC_AT91RM9200

As the ARCH_AT91RM9200 is removed because being !DT, we use
the SOC_AT91RM9200 variant. This option can certainly be removed
once the ST driver is reworked a bit.

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