pmdomains: core: fix unused variable warning with !PM_GENERIC_DOMAINS_OF

The genpd provider bus is really only used when
CONFIG_PM_GENERIC_DOMAINS_OF is enabled, and since the recent deferred
initialisation of domain parent devices, the root device pointer is
otherwise unused.

Fix the unused variable warning by moving the definition of the root
device pointer inside the corresponding ifdef.

Fixes: 92b69eff8012 ("pmdomain: fix early domain registration")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202606111746.kAxaAbwg-lkp@intel.com/
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ulf Hansson <ulfh@kernel.org>