naomi: lower rom dma transfer speed

Decrease from 20 to 7 MB/s. Fixes various freezes with Force Five aw
conversion.
Issue #141

Don't expose atomiswave A0_006 registers to naomi and other arcade
platforms.

core/hw/holly/sb_mem.cpp

@@ -106,8 +106,7 @@ T DYNACALL ReadMem_area0(u32 paddr)
 				else
 					return (T)0;
 			}
-			else
-			{
+			else if constexpr (System == DC_PLATFORM_ATOMISWAVE) {
 				return (T)libExtDevice_ReadMem_A0_006(addr, sz);
 			}
 		}
@@ -217,12 +216,12 @@ void DYNACALL WriteMem_area0(u32 paddr, T data)
 			{
 				if (!config::EmulateBBA)
 					ModemWriteMem_A0_006(addr, data, sz);
+				return;
 			}
-			else
-			{
+			else if constexpr (System == DC_PLATFORM_ATOMISWAVE) {
 				libExtDevice_WriteMem_A0_006(addr, data, sz);
+				return;
 			}
-			return;
 		}
 		// AICA sound registers
 		if (addr >= 0x00700000 && addr <= 0x00707FFF)

core/hw/naomi/naomi.cpp

@@ -159,8 +159,8 @@ static void Naomi_DmaStart(u32 addr, u32 data)
 		SB_GDST = 1;
 		// Max G1 bus rate: 50 MHz x 16 bits
 		// SH4_access990312_e.xls: 14.4 MB/s from GD-ROM to system RAM
-		// Here: 20 MB/s
-		sh4_sched_request(dmaSchedId, SB_GDLEN * 10);
+		// Here: 7 MB/s
+		sh4_sched_request(dmaSchedId, SB_GDLEN * 27);
 		return;
 	}
 	else