hw/sd/milkymist: Do not create SD card within the SD host controller

SD/MMC host controllers provide a SD Bus to plug SD cards,
but don't come with SD card plugged in :) Let the machine/board
model create and plug the SD cards when required.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200705211016.15241-5-f4bug@amsat.org>