ARM: configs: at91: sama7: Unselect CONFIG_DMATEST

The DMA test client should be selected only when one is debugging a DMA
Device driver. There's no need to select the DMA test client by default,
unselect it.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220228085906.18508-1-tudor.ambarus@microchip.com