of: property: Add fw_devlink support for optional properties

Not all DT bindings are mandatory bindings. Add support for optional DT
bindings and mark iommus, iommu-map, dmas as optional DT bindings.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20210205222644.2357303-5-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>