efi_loader: disk: a helper function to delete efi_disk objects

This function is expected to be called, in particular from dm's pre_remove
hook, when associated block devices no longer exist.

Add efi_disk_remove() function.
This function removes an efi_disk object for a raw disk device (UCLASS_BLK)
and related objects for its partitions (UCLASS_PARTITION).

So this function is expected to be called through driver model's "remove"
interface every time a raw disk device is to be disconnected.

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