xen: refactor physical address space compression support into common code

xen: refactor physical address space compression support into common code

The "pdx compression" functionality will be useful on ARM as well.

Move the code to common code+header and introduce HAS_PDX to control when it is
built. L2_PAGETABLE_SHIFT is x86 specific, so introduce PDX_GROUP_SHIFT to
abstract it out.

ARM has no need for superpage compression (yet?) and lacks SUPERPAGE_SHIFT so
those functions (spage_to_mfn et al) are not moved.

No affect on x86 and no change for ARM (yet).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>