of: Move reserved memory private function declarations

fdt_init_reserved_mem() and fdt_reserved_mem_save_node() are private to
the DT code, so move there declarations to of_private.h. There's no need
for the dummy functions as CONFIG_OF_RESERVED_MEM is always enabled for
CONFIG_OF_EARLY_FLATTREE.

Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210527193841.1284169-1-robh@kernel.org