arm: socfpga: Allow boards to define a custom environment size

This patch makes it possible that boards can define a board-specific env
size. This is used by the SR1500 SoCFPGA board port.

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