xen: Use typesafe gfn in xenmem_add_to_physmap_one

xen: Use typesafe gfn in xenmem_add_to_physmap_one

The x86 version of the function xenmem_add_to_physmap_one contains
variable name gpfn and gfn which make the code very confusing.
I have left unchanged for now.

Also, rename gpfn to gfn in the ARM version as the latter is the correct
acronym for a guest physical frame.

Finally, remove the trailing whitespace around the changes.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>