PSCI v0.2+ allows the kernel to probe the PSCI firmware version.

This patch replaces the default initialization of PSCI v0.2+
functions with code that allows probing PSCI firmware version
and initializes PSCI functions accordingly.

Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: NHanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>