firmware/psci: Add psci_early_test_conduit()

Add a function to test early if PSCI is present and what conduit it
uses. Because the PSCI conduit corresponds to the SMCCC one, this will
let the kernel know whether it can use SMC instructions to discuss with
the Realm Management Monitor (RMM), early enough to enable RAM and
serial access when running in a Realm.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Steven Price <steven.price@arm.com>
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v4: New patch