provide generic page_to_phys and phys_to_page implementations

[Christoph Hellwig]

page_to_phys is duplicated by all architectures, and from some strange
reason placed in <asm/io.h> where it doesn't fit at all.  

phys_to_page is only provided by a few architectures despite having a lot 
of open coded users.

Provide generic versions in <asm-generic/memory_model.h> to make these
helpers more easily usable.

Diffstat:
 arch/alpha/include/asm/io.h         |    1 -
 arch/arc/include/asm/io.h           |    3 ---
 arch/arm/include/asm/memory.h       |    6 ------
 arch/arm64/include/asm/memory.h     |    6 ------
 arch/csky/include/asm/page.h        |    3 ---
 arch/hexagon/include/asm/page.h     |    6 ------
 arch/loongarch/include/asm/page.h   |    3 ---
 arch/m68k/include/asm/virtconvert.h |    3 ---
 arch/microblaze/include/asm/page.h  |    1 -
 arch/mips/include/asm/io.h          |    5 -----
 arch/nios2/include/asm/io.h         |    3 ---
 arch/openrisc/include/asm/page.h    |    2 --
 arch/parisc/include/asm/page.h      |    1 -
 arch/powerpc/include/asm/io.h       |   12 ------------
 arch/riscv/include/asm/page.h       |    3 ---
 arch/s390/include/asm/page.h        |    2 --
 arch/sh/include/asm/page.h          |    1 -
 arch/sparc/include/asm/page.h       |    2 --
 arch/um/include/asm/pgtable.h       |    2 --
 arch/x86/include/asm/io.h           |    5 -----
 arch/xtensa/include/asm/page.h      |    1 -
 include/asm-generic/memory_model.h  |    3 +++
 22 files changed, 3 insertions(+), 71 deletions(-)