m68knommu: fix definitions of __pa() and __va()

Fix compilation breakage of all m68knommu targets:

  CC      arch/m68knommu/kernel/asm-offsets.s
In file included from include/linux/sched.h:77,
                 from arch/m68knommu/kernel/asm-offsets.c:12:
include/linux/percpu.h: In function 'per_cpu_ptr_to_phys':
include/linux/percpu.h:161: error: implicit declaration of function 'virt_to_phy

This is broken in linux-2.6.33-rc3.

Change the definitions of __pa() and __va() to not use virt_to_phys()
and phys_to_virt(). Trivial 1:1 conversion required for the non-MMU case.

A side effect if this is that the m68knommu can now use asm/virtconvert.h
for the definition of virt_to_phys() and phys_to_virt().

Also cleaned up the definition of page_to_phys() when moving into
virtconvert.h.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>