uboot-at91: use sdmmc0 as booting media for sama5d27_som1_ek

uboot-at91: use sdmmc0 as booting media for sama5d27_som1_ek

Commit adc69fe (""uboot-at91: changed som1 ek default defconfigs")
changed the booting media to sdmmc1 as default booting w/o any reason.
The Microchip releases for the rest of supported distributions (Buildroot,
Yocto Project) uses sdmmc0 as default booting media for this board.
Thus change it back to sdmmc0. With this remove references to sdmmc1
config. The initial commit cannot be cleanly reverted.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>