Reserved memory regions can be used for more than just DMA regions, so
only enabling on DMA_DECLARE_COHERENT (via HAS_DMA) or DMA_CMA is wrong.
This effectively doesn't matter except for the few cases arches select
NO_DMA.

At least, these users of RESERVEDMEM_OF_DECLARE depend on reserved memory
support:

arch/riscv/mm/init.c:RESERVEDMEM_OF_DECLARE(elfcorehdr, "linux,elfcorehdr", elfcore_hdr_setup);
drivers/memory/tegra/tegra210-emc-table.c:RESERVEDMEM_OF_DECLARE(tegra210_emc_table, "nvidia,tegra210-emc-table",
drivers/soc/fsl/qbman/bman_ccsr.c:RESERVEDMEM_OF_DECLARE(bman_fbpr, "fsl,bman-fbpr", bman_fbpr);
drivers/soc/fsl/qbman/qman_ccsr.c:RESERVEDMEM_OF_DECLARE(qman_fqd, "fsl,qman-fqd", qman_fqd);
drivers/soc/fsl/qbman/qman_ccsr.c:RESERVEDMEM_OF_DECLARE(qman_pfdr, "fsl,qman-pfdr", qman_pfdr);

Let's simplify things and enable OF_RESERVED_MEM when OF_EARLY_FLATTREE is
enabled.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210527223217.1572631-1-robh@kernel.org