Revert "rockchip: Remove unused rockchip_pd_pwr_down_wfi function"

This reverts commit b6dcbf58

.

This function wasn't used when it was removed, but it is needed to
compile the new changes proposed for Rockchip platforms.

Change-Id: Ia5bfe1f8398e08431f96923e2f059a83e5cb78d4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/rockchip/common/plat_pm.c

@@ -367,6 +367,15 @@ static void __dead2 rockchip_system_poweroff(void)
 	rockchip_soc_system_off();
 }
 
+static void __dead2 rockchip_pd_pwr_down_wfi(
+		const psci_power_state_t *target_state)
+{
+	if (RK_SYSTEM_PWR_STATE(target_state) == PLAT_MAX_OFF_STATE)
+		rockchip_soc_sys_pd_pwr_dn_wfi();
+	else
+		rockchip_soc_cores_pd_pwr_dn_wfi(target_state);
+}
+
 /*******************************************************************************
  * Export the platform handlers via plat_rockchip_psci_pm_ops. The rockchip
  * standard