ARM: configs: at91: sama7: Enable MTD_UBI_BLOCK

Address read only block filesystems like SquashFS, and use MTD_UBI_BLOCK
to benefit of UBI's bit-flip handling and wear-levelling.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220420134740.193563-3-tudor.ambarus@microchip.com