at91bootstrap: use sdmmc0 as booting media for sama5d27_som1_ek

at91bootstrap: use sdmmc0 as booting media for sama5d27_som1_ek

Commit 0b7c66c ("at91bootstrap: add sama5d27_som1_eksd1_uboot as
default defconfig") changed default booting media for sama5d27_som1_ek
board w/o any reason. Changed it back to sdmmc0 as it is for all the
other Microchip supported distributions for this board (Buildroot,
Yocto Project). The initial commit cannot be cleanly reverted.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
(cherry picked from commit e9f12931e60ee291cd7d2c8fd19a14682dae0197)