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?

Start by reviewing the SD library API and the existing SD.begin() usage shown in the issue. Compare the current SPI-mode API with the proposed native SDIO_PORT form and investigate how Arduino cores expose SDIO ports. Done means reaching agreement on an API design and documenting the required behavior; the issue provides no named files or tests.

Written by the indexing model from the issue text.