EmbeddedPkg: normalize mmc interaction latency to 1us

Upon inspecting the source code, I found that the original implementation
of eMMC timeout in MmcDxe is affected by the CPU execution speed. With
higher-performance CPUs, the actual timeout duration would be shortened as
it was not based on wall clock.

Add a precise 1us delay to each iteration of the wait loop to make the
actual duration of the delay clock- and performance-agnostic.

Signed-off-by: snowpiaoling <1972997989@qq.com>

EmbeddedPkg/Universal/MmcDxe/MmcBlockIo.c

@@ -210,6 +210,8 @@ MmcTransferBlock (
         break;  // Prevents delay once finished
       }
     }
+
+    gBS->Stall (1);
   }
 
   if (BufferSize > This->Media->BlockSize) {
@@ -323,6 +325,8 @@ MmcIoBlocks (
       if (!EFI_ERROR (Status)) {
         MmcHost->ReceiveResponse (MmcHost, MMC_RESPONSE_TYPE_R1, Response);
       }
+
+      gBS->Stall (1);
     }
 
     if (0 == Timeout) {

EmbeddedPkg/Universal/MmcDxe/MmcIdentification.c

@@ -596,6 +596,10 @@ MmcIdentificationMode (
       return Status;
     }
 
+    if (!OcrResponse.Ocr.PowerUp) {
+      gBS->Stall (1);
+    }
+
     Timeout--;
   } while (!OcrResponse.Ocr.PowerUp && (Timeout > 0));