of: declare string literals const

of_overlay_action_name() returns a string literal from a function local
array. Modifying string literals is undefined behavior which usage of
const pointer can avoid. of_overlay_action_name() is currently only
used once in overlay_notify() to print the returned value.

While on it declare the data array const as well.

Reported by Clang:

In file included from arch/x86/kernel/asm-offsets.c:22:
In file included from arch/x86/kernel/../kvm/vmx/vmx.h:5:
In file included from ./include/linux/kvm_host.h:19:
In file included from ./include/linux/msi.h:23:
In file included from ./arch/x86/include/asm/msi.h:5:
In file included from ./arch/x86/include/asm/irqdomain.h:5:
In file included from ./include/linux/irqdomain.h:35:
./include/linux/of.h:1555:3: error: initializing 'char *' with an expression of type 'const char[5]' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
"init",
^~~~~~
./include/linux/of.h:1556:3: error: initializing 'char *' with an expression of type 'const char[10]' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
"pre-apply",
^~~~~~~~~~~
./include/linux/of.h:1557:3: error: initializing 'char *' with an expression of type 'const char[11]' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
"post-apply",
^~~~~~~~~~~~
./include/linux/of.h:1558:3: error: initializing 'char *' with an expression of type 'const char[11]' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
"pre-remove",
^~~~~~~~~~~~
./include/linux/of.h:1559:3: error: initializing 'char *' with an expression of type 'const char[12]' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
"post-remove",
^~~~~~~~~~~~~

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20221012174622.45006-1-cgzones@googlemail.com
Signed-off-by: Rob Herring <robh@kernel.org>