pmdomain: core: introduce dev_pm_genpd_is_on()

This helper function returns the current power status of a given generic
power domain.

As example, remoteproc/imx_rproc.c can now use this function to check
the power status of the remote core to properly set "attached" or
"offline" modes.

Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Link: https://lore.kernel.org/r/20250629172512.14857-2-hiagofranco@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>