[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>