dbdma: support optinal CommandPtrHi register.

It's written with zero in OS X. This dummy implementation
is meant to reduce log warnings.

devices/common/dbdma.cpp

@@ -263,6 +263,9 @@ void DMAChannel::reg_write(uint32_t offset, uint32_t value, int size) {
         break;
     case DMAReg::CH_STAT:
         break; // ingore writes to ChannelStatus
+    case DMAReg::CMD_PTR_HI: // Mac OS X writes this optional register with zero
+        LOG_F(9, "CommandPtrHi set to 0x%X", value);
+        break;
     case DMAReg::CMD_PTR_LO:
         if (!(this->ch_stat & CH_STAT_RUN) && !(this->ch_stat & CH_STAT_ACTIVE)) {
             this->cmd_ptr = value;

devices/common/dbdma.h

@@ -39,6 +39,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 enum DMAReg : uint32_t {
     CH_CTRL         = 0,
     CH_STAT         = 4,
+    CMD_PTR_HI      = 8,
     CMD_PTR_LO      = 12,
     INT_SELECT      = 16,
     BRANCH_SELECT   = 20,