xen: arm: Use a direct mapping of RAM on arm64

xen: arm: Use a direct mapping of RAM on arm64

We have plenty of virtual address space so we can avoid needing to map and
unmap pages all the time.

A totally arbitrarily chosen 32GB frame table leads to support for 5TB of RAM.
I haven't tested with anything near that amount of RAM though. There is plenty
of room to expand further when that becomes necessary.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>