soc: mediatek: cmdq: add cmdq_dev_get_client_reg function

GCE cannot know the register base address, this function
can help cmdq client to get the cmdq_client_reg structure.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Houlong Wei <houlong.wei@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>