asm-generic: Introduce page.h

At the moment we are using 4K pages all the time,
so instead of copying code over all archs we're
supporting -- add asm-generic/page.h header.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>