MIPS: fw: arc: add __weak to prom_meminit and prom_free_prom_memory

As far as I understood, prom_meminit() in arch/mips/fw/arc/memory.c
is overridden by the one in arch/mips/sgi-ip32/ip32-memory.c if
CONFIG_SGI_IP32 is enabled.

The use of EXPORT_SYMBOL in static libraries potentially causes a
problem for the llvm linker [1]. So, I want to forcibly link lib-y
objects to vmlinux when CONFIG_MODULES=y.

As a groundwork, we must fix multiple definitions that have previously
been hidden by lib-y.

The prom_cleanup() in this file is already marked as __weak (because
it is overridden by the one in arch/mips/sgi-ip22/ip22-mc.c).
I think it should be OK to do the same for these two.

[1]: https://github.com/ClangBuiltLinux/linux/issues/515

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-By: Thomas Bogendoerfer <tsbogend@alpha.franken.de>