xen/arm: Rework the interface of p2m_cache_flush and use typesafe gfn

xen/arm: Rework the interface of p2m_cache_flush and use typesafe gfn

p2m_cache_flush is expecting GFNs in parameter and not MFNs. Rename
the variable to *gfn* and use typesafe to avoid possible misusage.

Also, modify the prototype of the function to describe the range
using the start and the number of GFNs. This will avoid to wonder
whether the end if inclusive or exclusive.

Note that the type of the parameters 'start' is changed from xen_pfn_t
(aka uint64_t) to gfn_t (aka unsigned long). This means that a truncation
will occur for ARM32. It is fine because it will always be encoded on 28
bits maximum (40 bits address).

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