[IA64] Rationalise Region Definitions

Currently, region numbers are defined in several files, with several 
names.  For example, we have REGION_KERNEL in asm/page.h and 
RGN_KERNEL in pgtable.h 
 
We also have address definitions that should depend on the 
RGN_XXX macros, but are currently just long constants. 
 
The following patch reorganises all the definitions so that they have 
the same form (RGN_XXX), are in one place, and that addresses that 
depend on RGN_XXX are derived from them. 

(This is a necessary but not sufficient patch to allow UML-like 
operation on IA64). 

Thanks to David Mosberger for catching the change I missed in mmu_context.h.
 
Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>