arm: socfpga: Zap checkboard()

Since all boards now have a DT, instead of hard-coding the board
name into the U-Boot binary, read the board name from DT "model"
property.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@opensource.altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Pavel Machek <pavel@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Cc: Vince Bridgers <vbridger@opensource.altera.com>