configs/socrates_cyclone5: new defconfig

Configuration contains:
  - building uboot (and spl) from altera's github
  - booting from SD card
  - working ethernet, serial, i2c, fpga_manager
  - minimal rootfs with busybox
  - ready to flash SD card image

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>
[Thomas:
 - add entry to DEVELOPERS file
 - refresh .gitlab-ci.yml
 - fix commit title]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>