of: make struct property _flags field configurable

Only Sparc and CONFIG_OF_DYNAMIC use the struct property._flags field,
so make it conditional shrinking struct property a bit.

Tested-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <robh@kernel.org>