arm: omap: rename board-rx51-sdram.c to sdram-nokia.c

Rename the file and functions so that it can be reused by future Nokia
boards. Based on patches by Juha Keski-Saari.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>