arm64: allow ioremap_cache() to work with RAM

Some drivers call ioremap_cache() to map an address which could either
be outside of RAM or in a reserved area of RAM. We can't normally ioremap
RAM, but since the driver wants a cacheable mapping, we can just use
the existing kernel mapping of RAM in this particular case.

Signed-off-by: Mark Salter <msalter@redhat.com>