Add totalBytes / usedBytes to SD/SDMMC (#673)

to be same as SPIFFS

libraries/SD_MMC/examples/SDMMC_Test/SDMMC_Test.ino

@@ -209,6 +209,8 @@ void setup(){
     renameFile(SD_MMC, "/hello.txt", "/foo.txt");
     readFile(SD_MMC, "/foo.txt");
     testFileIO(SD_MMC, "/test.txt");
+    Serial.printf("Total space: %lluMB\n", SD_MMC.totalBytes() / (1024 * 1024));
+    Serial.printf("Used space: %lluMB\n", SD_MMC.usedBytes() / (1024 * 1024));
 }
 
 void loop(){