UPSTREAM: KVM: arm64: Define FGT nMASK bits relative to other fields

Now that RES0 and MASK have full coverage, no need to manually
encode nMASK. Calculate it relative to the other fields.

No functional change intended.

Bug: 278749606
Change-Id: Ic0ab29d18f12d93b68198d36733720f78c9a443f
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231214100158.2305400-14-tabba@google.com
(cherry picked from commit 5f6bd3f3)