rockchip: Remove unused rockchip_pd_pwr_down_wfi function

The rockchip_pd_pwr_down_wfi function is currently unused, which may
trigger compiler warnings or errors. Remove it.

Change-Id: I7e1b0ae092e8855528ac2065ecefc8bd45305f31
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>