ARM: configs: at91: sama7: Enable MTD_UBI_FASTMAP

Use fastmap to accelerate the attach procedure.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
[nicolas.ferre@microchip.com: removed unrelated changes]
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220420134740.193563-2-tudor.ambarus@microchip.com