dm: blk: Add a function to find an interface-type name

dm: blk: Add a function to find an interface-type name

Add a function to find the name of an interface type (e.g. "sata", "scsi")
from the interface type enum.

This is useful for generic code (not specific to SATA or SCSI, for
example) that wants to display the type of interface it is dealing with.

Signed-off-by: Simon Glass <sjg@chromium.org>