Update memory.cpp

Update memory.cpp

Novel changes that do not affect emulation.  Allows 32-bit ROM to be used with AGA chipset when using "KS ROM has Chip RAM speed" custom chipset configuration.

Also, a default "checkerboard" pattern is written to RAM on hard reset, to mimic the column/row pattern typically found in DRAM chips on power cycle.  Random noise in DRAM chips is not emulated to ensure uninitialized memory will compress well when storing a save state.