of: reserved-memory: ignore disabled memory-region nodes

Ignore disabled nodes in the memory-region
nodes list and continue to initialize the rest
of enabled nodes.

Check if the "reserved-memory" node is available
and if it's not available, return 0 to ignore the
"reserved-memory" node and continue parsing with
next node in memory-region nodes list.

Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Signed-off-by: Puneet Saxena <puneets@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>