Get internal SRAM working (really)

Need to assert SZRQn to enable reads at A[1]=1

Now the BIOS decides the memory is writable, and checksums the first
128 bytes (twice).

And then it hits a DIV instruction and throws an exception.

rtl/fx_ga.sv

@@ -76,7 +76,7 @@ logic           unk_cen;
 logic           io_readyn;
 
 assign READYn = unk_cen & ROM_READYn & RAM_READYn & SRAM_READYn & io_readyn;
-assign SZRQn = ~unk_cen | (ROM_CEn & IO_CEn);
+assign SZRQn = ~unk_cen | (ROM_CEn & IO_CEn & SRAM_CEn);
 
 //////////////////////////////////////////////////////////////////////
 // Address decoder