UPSTREAM: KVM: arm64: Use generated FGT RES0 bits instead of specifying them

Now that all FGT fields are accounted for and represented, use
the generated value instead of manually specifying them.

For __HFGWTR_EL2_RES0, however, there is no generated value. Its
fields are subset of HFGRTR_EL2, with the remaining being RES0.
Therefore, add a mask that represents the HFGRTR_EL2 only bits
and define __HFGWTR_EL2_* using those and the __HFGRTR_EL2_*
fields.

No functional change intended.

Bug: 278749606
Change-Id: I246926bf7d0800c0285be1823ba2e19d055e4315
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231214100158.2305400-13-tabba@google.com
(cherry picked from commit 9ff67dd2)