sparc: move page_to_phys to page.h

Preparation for introducing asm-generic/io.h this move was required.
In asm-generic page_to_phys is placed in page.h - so do the same here.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>