arm: at91: gardena-smart-gateway-at91sam: Enable CONFIG_SYS_NAND_USE_FLASH_BBT

This patch enables the BBT in NAND on the AT91SAM based GARDENA smart
Gateway. This is especially important, since the Linux driver also
enables this option and uses the BBT table pages. Without setting this
option, U-Boot will try to re-use these pages again (e.g. UBI).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Tom Rini <trini@konsulko.com>