RFC: switch the remaining architectures to use generic GUP

[Christoph Hellwig]

Hi Linus and maintainers,

below is a series to switch mips, sh and sparc64 to use the generic
GUP code so that we only have one codebase to touch for further
improvements to this code.  I don't have hardware for any of these
architectures, and generally no clue about their page table
management, so handle with care.  But it at least survives a
basic defconfig compile test..