sandbox: mmc: Support fixed MMC devices

Add support for reading devicetree flags for MMC devices. With this we
can distinguish between fixed and removable drives. Note that this
information is only available when the device is probed, not when it is
bound, since it is read in the of_to_plat() method. This could be changed
if needed later.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>