atmel-mci: Platform code for supporting multiple mmc slots

Add the necessary platform infrastructure to support multiple mmc/sdcard
slots all at once through a single controller. Currently, the driver
will use the first valid slot it finds and stick with that, but later
patches will add support for switching between several slots on the fly.

Extend the platform data structure with per-slot information: MMC/SDcard
bus width and card detect/write protect pins. This will affect the pin
muxing as well as the capabilities announced to the mmc core.

Note that board code is now required to supply a mci_platform_data
struct to at32_add_device_mci().

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>