asm-generic cleanups for 6.8

A series from Baoquan He cleans up the asm-generic/io.h to remove the
ioremap_uc() definition from everything except x86, which still needs it
for pre-PAT systems. This series notably contains a patch from Jiaxun Yang
that converts MIPS to use asm-generic/io.h like every other architecture
does, enabling future cleanups.

Some of my own patches fix -Wmissing-prototype warnings in architecture
specific code across several architectures. This is now needed as the
warning is enabled by default. There are still some remaining warnings
in minor platforms, but the series should catch most of the widely used
ones make them more consistent with one another.

David McKay fixes a bug in __generic_cmpxchg_local() when this is used
on 64-bit architectures. This could currently only affect parisc64
and sparc64.

Additional cleanups address from Linus Walleij, Uwe Kleine-König,
Thomas Huth, and Kefeng Wang help reduce unnecessary inconsistencies
between architectures.
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