drivers: of: add definition of early_init_dt_alloc_reserved_memory_arch

The function early_init_dt_alloc_reserved_memory_arch is defined
in drivers/of/of_reserved_mem.c but is not declared in any of the
header files. Add the declaration of this to avoid the warning:

drivers/of/of_reserved_mem.c:31:19: warning: symbol 'early_init_dt_alloc_reserved_memory_arch' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[robh: drop extern from declaration]
Signed-off-by: Rob Herring <robh@kernel.org>