firmware/psci: use common SMCCC_CONDUIT_*

Now that we have common SMCCC_CONDUIT_* definitions, migrate the PSCI
code over to them, and kill off the old PSCI_CONDUIT_* definitions.

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

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>