soc: ti: Add ti_sci_pm_domains driver

Introduce a ti_sci_pm_domains driver to act as a generic pm domain
provider to allow each device to attach and associate it's ti-sci-id so
that it can be controlled through the TI SCI protocol.

This driver implements a simple genpd where each device node has a
phandle to the power domain node and also must provide an index which
represents the ID to be passed with TI SCI representing the device using
a single phandle cell. The driver manually parses the phandle to get the
cell value. Through this interface the genpd dev_ops start and stop
hooks will use TI SCI to turn on and off each device as determined by
pm_runtime usage.

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>