Partially Revert "of: fix sparse warnings in fdt, irq, reserved mem, and resolver code"

A change to function pointers that was meant to address a sparse warning
turned out to cause hundreds of new gcc-7 warnings:

include/linux/of_irq.h:11:13: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
drivers/of/of_reserved_mem.c: In function '__reserved_mem_init_node':
drivers/of/of_reserved_mem.c:200:7: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
   int const (*initfn)(struct reserved_mem *rmem) = i->data;

Turns out the sparse warnings were spurious and have been fixed in
upstream sparse since 0.5.0 in commit "sparse: treat function pointers
as pointers to const data".

This partially reverts commit 17a70355.

Fixes: 17a70355 ("of: fix sparse warnings in fdt, irq, reserved mem, and resolver code")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <robh@kernel.org>