ARM: psci: add probe function to discover presence of a psci implementation

ARM: psci: add probe function to discover presence of a psci implementation

This patch adds a probe function to check if the secure firmware has an
implementation of the Power State Coordination Interface.

'bL_platform_power_ops' will be implemented by:

a. a native backend when Linux runs in secure world
b. a psci backend which relies on the secure firmware to implement the
   power ops

presence of b. will be indicated by the psci device node in the device tree.
The device node is expected to be populated by the secure firmware if it
supports psci. If the native backend detects a psci node then it bails out
allowing the psci backend to be registered.

Also a dummy 'psci_probe' function is added for the case when psci support
is not included. This prevents the build from breaking for tc2 and the
rtsm platforms.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>