arm: Tidy up flush_xen_dcache().

arm: Tidy up flush_xen_dcache().

 - Use a compile-time-constant check for whether we can safely flush
   just one cacheline.  This reduces the common case from 28
   instructions to three.
 - Pass an object to the macro, not a pointer, so we can detect
   attempts to flush arrays.
 - Decode CCSIDR correctly to get cacheline size.
 - Remove some redundant DSBs at the call sites.

Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>