firmware: psci: register PSCI power domains to stub driver

firmware: psci: register PSCI power domains to stub driver

On some SoCs, like Qualcomm SoCs, the PSCI cluster power domain
is used by system-wide firmware interfaces to make sure none
of the CPUs are suspended before submitting requests.

While on U-boot we only use the first core and we never
suspend it, the Device Tree still references it and blocks
those nodes to be probed.

Simply bind the PSCI power-domain subnoded to a stub power
domain driver in order to solve the runtime dependencies.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>