ARM: at91: remove useless config MACH_AT91RM9200_DT

Now that rm9200 is only supported through DT, remove CONFIG_MACH_AT91RM9200_DT
and use CONFIG_SOC_AT91RM9200 instead.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>