SD library API for use with SDIO port

All SD cards support 2 access methods, native SDIO mode and a simpler/slower SPI mode meant for microcontrollers. Some newer 32 bit microcontrollers are featuring native SDIO ports. I'm working with one right now....

Would like any input on how the SD library API should be extended to support these? So far, my main thought is to define a symbol like SDIO_PORT

```
SD.begin(4); // use SPI mode with pin 4 as chip select

SD.begin(SDIO_PORT); // use native SDIO port, which does not have a chip select line
```

Does this look reasonable? Or has anyone already made other plans or ideas for supporting chips with native SDIO?

首先审查 SD library API 以及 issue 中展示的现有 SD.begin() 用法。将当前的 SPI-mode API 与提议的原生 SDIO_PORT 形式进行比较，并调查 Arduino cores 如何公开 SDIO 端口。达成 API 设计共识并记录所需行为即视为完成；issue 未提供指定的文件或测试。

由索引模型根据 Issue 内容生成。