board: Add support for Sielaff i.MX6 Solo board

The Sielaff i.MX6 Solo board is a control and HMI board for vending
machines. Add support for this board.

The devicetree files are taken from pending changes in the Linux
kernel that are available from linux-next and will likely be
part of Linux v6.9.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>