xen/arm: remove outer_*_range call

Dom0 is not actually capable of issuing outer_inv_range or
outer_clean_range calls.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>