sunxi: psci: Fix sunxi_power_switch on sun8i-r40 platform

linux system will die if we offline one of the cpu on R40 based board:
eg: echo 0 > /sys/devices/system/cpu/cpu3/online

The reason is that the R40 version of sunxi_cpu_set_power always passes
0 for the CPU number, so we turn off CPU0, regardless of what CPU the
CPU_OFF request came for.

Fix this by passing the proper CPU number, as there are proper power
clamp registers for every of the four cores.

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>