configs: rename atmel_xplained_defconfig

For consistency, rename atmel_xplained_defconfig to
atmel_sama5d3_xplained_mmc_defconfig as this configuration supports
booting the sama5d3 Xplained with an MMC card (as opposed to the NAND
flash).

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>