- Moved SDIO-related structures into their own file;
- Added IVAR access methods for retrieving SDIO vendor and product info;
- Added code for initializing child devices.
- Added MMC_IVAR_SDIO_VENDOR, MMC_IVAR_SDIO_PRODUCT and accessors for them.
- Added a dummy sceleton for Marvell Wifi. Need this to test my SDIO stuff.
- Added bus method: mmcbus_io_f0_read_1.

This allows reading individual configuration registers from SDIO function 0.
- Added MMC_IVAR_SDIO_FUNCTION for accessing function number;
- Added mmcb_io_read_1 to read one byte from the current function's space.
- Remove garbage accidentally committed previously.