of/fdt: add of_fdt_device_is_available function

Add an equivalent function to of_device_is_available for flattened DT, and
convert the one existing open coded occurrence.

Tested-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>