asm-generic: provide generic page_to_phys and phys_to_page implementations

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.

Note with this patch powerpc loses the CONFIG_DEBUG_VIRTUAL pfn_valid
check.  It will be added back in a generic version later.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>