ARM: KVM: fix fault_ipa computing

The ARM ARM says that HPFAR reports bits [39:12] of the faulting
IPA, and we need to complement it with the bottom 12 bits of the
faulting VA.

This is always 12 bits, irrespective of the page size. Makes it
clearer in the code.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>