of: Move all FDT reserved-memory handling into of_reserved_mem.c

The split of /reserved-memory handling between fdt.c and
of_reserved_mem.c makes for reading and restructuring the code
difficult. As of_reserved_mem.c is only built for
CONFIG_OF_EARLY_FLATTREE already, move all the code to one spot.

Acked-by: Saravana Kannan <saravanak@google.com>
Reviewed-by: Oreoluwa Babatunde <quic_obabatun@quicinc.com>
Link: https://lore.kernel.org/r/20240311181303.1516514-2-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>