hw/sd: Move sdcard legacy API to 'hw/sd/sdcard_legacy.h'

omap_mmc.c is the last device left using the legacy sdcard API.
Move the prototype declarations into a separate header, to
make it clear this is a legacy API.

Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <20180216022933.10945-8-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
4 files changed