rockchip: Remove unused rockchip_pd_pwr_down_wfi function
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 <1a73af9e7ae00182733b2292511b814be66f065f@paulk.fr>

achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware