xen: introduce arch_grant_(un)map_page_identity

xen: introduce arch_grant_(un)map_page_identity

Introduce two arch specific functions to create a new p2m mapping of
granted pages at pfn == mfn.
We don't an x86 implementation as these functions should never be
compiled on x86 (they are called from an if (0) statement).

Base the implementation of arm_smmu_(un)map_page on
arch_grant_(un)map_page_identity.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>