efi_loader: disk: a helper function to create efi_disk objects from udevice

Add efi_disk_probe() function.
This function creates an efi_disk object for a raw disk device (UCLASS_BLK)
and additional objects for related partitions (UCLASS_PARTITION).

So this function is expected to be called through driver model's "probe"
interface every time one raw disk device is detected and activated.
We assume that partition devices (UCLASS_PARTITION) have been created
when this function is invoked.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>