MM58167 register index 0x0 and 0x5 are also half-registers

Related to https://github.com/86Box/86Box/issues/3347

0x0 and 0x8 low nibbles are always 0
0x5 and 0xd high nibbles are always 0

src/device/isartc.c

@@ -410,6 +410,7 @@ mm67_read(uint16_t port, void *priv)
             break;
 
         case MM67_AL_MSEC:
+        case MM67_MSEC:
             ret                = dev->nvr.regs[reg] & 0xf0;
             break;
 
@@ -417,6 +418,10 @@ mm67_read(uint16_t port, void *priv)
             ret                = dev->nvr.regs[reg] & 0x0f;
             break;
 
+        case MM67_DOW:
+            ret                = dev->nvr.regs[reg] & 0x07;
+            break;
+
         default:
             ret = dev->nvr.regs[reg];
             break;