pmdomain: core: add dummy release function to genpd device

The genpd device, which is really only used as a handle to lookup
OPP, but not even registered to the device core otherwise and thus
lifetime linked to the genpd struct it is contained in, is missing
a release function. After b8f7bbd1 ("pmdomain: core: Add
missing put_device()") the device will be cleaned up going through
the driver core device_release() function, which will warn when no
release callback is present for the device. Add a dummy release
function to shut up the warning.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Fixes: b8f7bbd1 ("pmdomain: core: Add missing put_device()")
Cc: stable@vger.kernel.org
Message-ID: <20241218184433.1930532-1-l.stach@pengutronix.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>