drivers: psci: Allow PSCI node to be disabled

Allow disabling PSCI support (mostly for testing purposes) by setting
the status property to "disabled". This makes the node behave in much
the same way as proper device nodes.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>