fdtdec: Support reserved-memory flags

Reserved memory nodes can have additional flags. Support reading and
writing these flags to ensure that reserved memory nodes can be properly
parsed and emitted.

This converts support for the existing "no-map" flag to avoid extending
the argument list for fdtdec_add_reserved_memory() to excessive length.

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