mm: split out mfn_t / gfn_t / pfn_t definitions and helpers

mm: split out mfn_t / gfn_t / pfn_t definitions and helpers

xen/mm.h has heavy dependencies, while in a number of cases only these
type definitions are needed. This separation then also allows pulling in
these definitions when including xen/mm.h would cause cyclic
dependencies.

Replace xen/mm.h inclusion where possible in include/xen/. (In
xen/iommu.h also take the opportunity and correct the few remaining
sorting issues.)

While the change could be dropped, remove an unnecessary asm/io.h
inclusion from xen/arch/x86/acpi/power.c. This was the initial attempt
to address build issues with it, until it became clear that the header
itself needs adjustment.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>