ddr: altera: sdram: Introduce socfpga_sdram_get_config()

Introduce socfpga_sdram_get_config() function implement in a board file,
which returns the socfpga_sdram_config structure. This is the last step
in cleaning up the socfpga_mmr_init_full(), but not the last step which
allows removing the inclusion of sdram.h from drivers/ddr/altera/sdram.c
thus far.

Signed-off-by: Marek Vasut <marex@denx.de>