ARM: uniphier: enable PSCI sysreset for uniphier_v8_defconfig

This configuration is supposed to be used with ARM Trusted Firmware,
so the SYSTEM_RESET is implemented in BL31.  Invoke PSCI instead of
U-Boot's own reset code because we need to coordinate with SCP
(System Control Processor) for the system-level power management.

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