xen/arm: groundwork for mem_access support on ARM

xen/arm: groundwork for mem_access support on ARM

Add necessary changes for page table construction routines to pass
the default access information and hypercall continuation mask. Also,
define necessary functions and data fields to be used later by mem_access.

The p2m_access_t info will be stored in a Radix tree as the PTE lacks
enough software programmable bits, thus in this patch we add the radix-tree
construction/destruction portions. The tree itself will be used later
by mem_access.

Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>