arm64: dts: uniphier: switch over to PSCI enable method

At the first system bring-up, I chose to use spin-table because ARM
Trusted Firmware was not ready for this platform at that moment.

Actually, these SoCs are equipped with EL3 and able to provide PSCI.
Now I finished porting the ATF BL31 for the UniPhier platform, so it
is ready to migrate to PSCI enable method.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>