dm: core: add functions to get/remap I/O addresses by name

This functions allow us to get and remap I/O addresses by name, which is useful when there are multiple reg addresses indexed by reg-names property.
This is needed in bmips dma/eth patch series, but can also be used on many
other drivers.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>