Add SDRAM controller (controller)
This adds an SDRAM controller which allows the 6502 to access SDRAM as if it were regular SRAM.
Turns out there are some issues with holding the chip select for the SDRAM controller high for too long, so there is a simple 2-state fsm which ensures that the chip select is only held for 1 clock cycle for writes and for as long as it takes to read the data from sdram for reads.
