of: select OF_RESERVED_MEM automatically

The OF_RESERVED_MEM can be used if we have either CMA or the generic
declare coherent code built and we support the early flattened DT.

So don't bother making it a user visible options that is selected
by most configs that fit the above category, but just select it when
the requirements are met.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rob Herring <robh@kernel.org>