Currently the sandbox block driver uses priv_auto_alloc_size for
the driver data, however that's only available after the device
probe phase. In order to make it accessible in an earlier phase,
switch to use platdata_auto_alloc_size instead.

This patch is the prerequisite for the follow up patch of DM BLK
driver changes to work with Sandbox.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>