hw/sd/ssi-sd: Do not create SD card within controller's realize

hw/sd/ssi-sd: Do not create SD card within controller's realize

ssi_sd_realize() creates an "sd-card" device.  This is inappropriate,
and marked FIXME.

Move it to the boards that create these devices.  Prior art: commit
eb4f566bbb for device "generic-sdhci", and commit 26c607b86b for
device "pl181".

The device remains not user-creatable, because its users should (and
do) wire up its GPIO chip-select line.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Bin Meng <bin.meng@windriver.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Cc: qemu-arm@nongnu.org
Cc: qemu-riscv@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-2-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>