ARM: at91: remove useless config MACH_AT91SAM9_DT

Now that at91sam9 SoCs are only supported through DT, remove
CONFIG_MACH_AT91SAM9_DT and use CONFIG_SOC_AT91SAM9 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>