xen: make include/xen/page.h portable moving those definitions under asm dir

The definitions in include/asm/xen/page.h are arch specific.
ia64/xen wants to define its own version. So move them to arch specific
directory and keep include/xen/page.h in order not to break compilation.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>