ddr: altera: sequencer: Wrap IO_* macros

Introduce structure socfpga_sdram_io_config to wrap the IO configuration
values in board file. Introduce socfpga_get_sdram_io_config() function,
which returns this the structure. This is another step toward wrapping
the nasty QTS generated macros in board files and reducing the pollution
of the namespace.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>