amic: implement diagnostics register.

devices/amic.cpp

@@ -89,6 +89,8 @@ uint32_t AMIC::read(uint32_t reg_start, uint32_t offset, int size)
         return this->snd_out_ctrl;
     case AMICReg::Snd_Out_DMA:
         return this->snd_out_dma->read_stat();
+    case AMICReg::Diag_Reg:
+        return 0xFFU; // this value allows the machine to boot normally
     default:
         LOG_F(WARNING, "Unknown AMIC register read, offset=%x", offset);
     }

devices/amic.h

@@ -99,6 +99,9 @@ enum AMICReg : uint32_t {
 
     Int_Ctrl            = 0x2A000,
 
+    // Undocumented diagnostics register
+    Diag_Reg            = 0x2C000,
+
     // DMA control registers
     DMA_Base_Addr_0     = 0x31000,
     DMA_Base_Addr_1     = 0x31001,