cpuidle: psci: Transition to the faux device interface

cpuidle: psci: Transition to the faux device interface

The PSCI cpuidle driver does not require the creation of a platform
device. Originally, this approach was chosen for simplicity when the
driver was first implemented.

With the introduction of the lightweight faux device interface, we now
have a more appropriate alternative. Migrate the driver to utilize the
faux bus, given that the platform device it previously created was not
a real one anyway. This will simplify the code, reducing its footprint
while maintaining functionality.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://patch.msgid.link/20250317-plat2faux_dev-v1-1-5fe67c085ad5@arm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>