hw/sd/pl181: Do not create SD card within the SD host controller

hw/sd/pl181: 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>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200705204630.4133-8-f4bug@amsat.org>