ddr: altera: sequencer: add RW_MGR_MEM_NUMBER_OF_RANKS

Fix build error for socfpga_cyclone5_defconfig:

board/altera/socfpga/wrap_sdram_config.c:245:26: error: ‘RW_MGR_MEM_NUMBER_OF_RANKS’ undeclared here (not in a function)
make[2]: *** [spl/board/altera/socfpga/wrap_sdram_config.o] Error 1

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>