KVM: arm64: Expose ID_AA64FPFR0_EL1 as a writable ID reg

ID_AA64FPFR0_EL1 contains all sort of bits that contain a description
of which FP8 subfeatures are implemented.

We don't really care about them, so let's just expose that register
and allow userspace to disable subfeatures at will.

Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240820131802.3547589-7-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/sys_regs.c

@@ -2386,7 +2386,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
 	ID_HIDDEN(ID_AA64SMFR0_EL1),
 	ID_UNALLOCATED(4,6),
-	ID_UNALLOCATED(4,7),
+	ID_WRITABLE(ID_AA64FPFR0_EL1, ~ID_AA64FPFR0_EL1_RES0),
 
 	/* CRm=5 */
 	{ SYS_DESC(SYS_ID_AA64DFR0_EL1),