of: reserved-memory: Support multiple regions per device

While the lookup/initialization code already supports multiple memory
regions per device, the release code will only ever release the first
matching memory region.

Enhance the code to release all matching regions. Each attachment of
a region to a device is uniquely identifiable using a struct device
pointer and a pointer to the memory region's struct reserved_mem.

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