FPGA: fix BSX PSRAM mapping
Correct LoROM SRAM mapping for smaller ROMs

SRAM is sometimes mapped not just to 70:0000-7fff but also to
70:8000-ffff if ROM size permits it (i.e. the ROM is small enough
to avoid overlap).
Map SRAM to 8000-ffff if the ROM mask denotes a ROM <= 16 MBits.