ARM: sunxi: select ARM_PSCI

On Allwinner sun7i (A20) SoCs, the SMP operations are implemented in
u-boot, and exposed to the kernel through PSCI.

We thus need to enable PSCI support for Allwinner SoCs so that the
kernel uses it to bring up the additionnal cores.

Signed-off-by: Signed-off-by: Zalan Blenessy <zalan.blenessy@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>