arm: socfpga: Introduce common board code

The SoCFPGA has reached a point where every single board code become
the same, since each and every single board is probed equally from OF.
Move the common board code into arch/arm/mach-socfpga/ .

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