sdhci: refactor common sysbus/pci class_init() into sdhci_common_class_init()

Now both inherited classes appear as DEVICE_CATEGORY_STORAGE.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180115182436.2066-5-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>