This removes the ram from inside the FPGA. All RAM is now located in the
external SDRAM instead.
The ROM is still in the FPGA to allow easier programming.
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.
This also increases the number of registers to 4, one more for the high
pair of displays, and a final one for a mask register which has not been
implemented yet.
