of: reserved-memory: Support lookup of regions by name

Add support for looking up memory regions by name. This looks up the
given name in the newly introduced memory-region-names property and
returns the memory region at the corresponding index in the memory-
region(s) property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>