soc: mediatek: fix the mistaken pointer accessed when subdomains are added

Fix the pointer to struct scp_subdomian not being moved forward
when each sub-domain is expected to be iteratively added through
pm_genpd_add_subdomain call.

Cc: stable@vger.kernel.org
Fixes: 53fddb1a ("soc: mediatek: reduce code duplication of scpsys_probe across all SoCs")
Reported-by: Weiyi Lu <weiyi.lu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>