fdtdec: Support compatible string list for reserved memory

Reserved memory nodes can have a compatible string list to identify the
type of reserved memory that they represent. Support specifying an
optional compatible string list when creating these nodes.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>