ddr: altera: sequencer: Wrap RW_MGR_* macros

Introduce structure socfpga_sdram_rw_mgr_config to wrap the RW manager
configuration values in board file. Introduce a complementary function,
socfpga_get_sdram_rwmgr_config(), 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>