imx: imx6ul: Add support for Kontron Electronics SL/BL i.MX6UL/ULL boards (N63xx...

imx: imx6ul: Add support for Kontron Electronics SL/BL i.MX6UL/ULL boards (N63xx/N64xx)

This adds support for i.MX6UL/ULL-based evaluation kits with SoMs by
Kontron Electronics GmbH.

Currently there are the following SoM flavors (SoM-Line):
  * N6310: SOM with i.MX6UL-2, 256MB RAM, 256MB SPI NAND
  * N6311: SOM with i.MX6UL-2, 512MB RAM, 512MB SPI NAND
  * N6411: SOM with i.MX6ULL, 512MB RAM, 512MB SPI NAND

And the according evaluation boards (Board-Line):
  * N6310-S: Baseboard with SOM N6310, eMMC, display (optional), ...
  * N6311-S: Baseboard with SOM N6311, eMMC, display (optional), ...
  * N6411-S: Baseboard with SOM N6411, eMMC, display (optional), ...

Currently U-Boot describes i.MX6UL and i.MX6ULL through separate config
options at compile-time. Though the differences are so minor, that for
the scope of these SoMs we just use a single defconfig that is compatible
with both SoCs.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>