efi_loader: disk: use udevice instead of blk_desc

In most of all cases, we can avoid using blk_desc which is expected
to be private to udevice(UCLASS_BLK), that is, the data should not
be manipulated outside the device driver unless really needed.

Now efi_disk's internally use dev_read/write() interfaces
if CONFIG_PARTITIONS is enabled.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>