Add a SPI SD/MMC card model. Write command not implemented yet.
Known to work with the TSAR SPI boot loader and NetBSD's sdmmc driver.
The data are backed by a file in the host's filesystem, as vci_block_device
does.
The purpose of this model is to provide a sdmmc device to the vci_spi
controller. It's not intended to respect timings or a real SD/MMC device.