the low power version have a mmc-spi

eanble mmc-spi and RV3029C2 RTC in at91_dt_defconfig
Signed-off-by: NJean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>