arm64: errata: use arm_smccc_1_1_get_conduit()

Now that we have arm_smccc_1_1_get_conduit(), we can hide the PSCI
implementation details from the arm64 cpu errata code, so let's do so.

As arm_smccc_1_1_get_conduit() implicitly checks that the SMCCC version
is at least SMCCC_VERSION_1_1, we no longer need to check this
explicitly where switch statements have a default case, e.g. in
has_ssbd_mitigation().

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>