efi_driver: blk: Switch to use platdata_auto_alloc_size for the driver data

Currently the efi 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 EFI loader.

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