booke/powerpc: define wimge shift mask to fix compilation error

This fixes below compilation error on SOCs where CONFIG_PHYS_64BIT
is not defined:

 arch/powerpc/kvm/e500_mmu_host.c: In function 'kvmppc_e500_shadow_map':
| arch/powerpc/kvm/e500_mmu_host.c:631:20: error: 'PTE_WIMGE_SHIFT' undeclared (first use in this function)
|    wimg = (*ptep >> PTE_WIMGE_SHIFT) & MAS2_WIMGE_MASK;
|                     ^
| arch/powerpc/kvm/e500_mmu_host.c:631:20: note: each undeclared identifier is reported only once for each function it appears in
| make[1]: *** [arch/powerpc/kvm/e500_mmu_host.o] Error 1

Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>