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 <[email protected]>
Cc: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Vince Bridgers <[email protected]>