pmdomain: ti-sci: Fix duplicate PD referrals

When the dts file has multiple referrers to a single PD (e.g.
simple-framebuffer and dss nodes both point to the DSS power-domain) the
ti-sci driver will create two power domains, both with the same ID, and
that will cause problems as one of the power domains will hide the other
one.

Fix this checking if a PD with the ID has already been created, and only
create a PD for new IDs.

Fixes: efa5c01c

 ("soc: ti: ti_sci_pm_domains: switch to use multiple genpds instead of one")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240415-ti-sci-pd-v1-1-a0e56b8ad897@ideasonboard.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>