arm: socfpga: spl: Add stub sdram.h

arm: socfpga: spl: Add stub sdram.h

Since the SoCFPGA SDRAM support is not yet applied to u-boot, we still
need to be able to compile the codebase. Introduce stub functions which
temporarily supplement the missing SDRAM setup functions.

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