sysreset: psci: support system reset in a generic way with PSCI

If the system is running PSCI firmware, the System Reset function
(func ID: 0x80000009) is supposed to be handled by PSCI, that is,
the SoC/board specific reset implementation should be moved to PSCI.
U-Boot should call the PSCI service according to the arm-smccc
manner.

The arm-smccc is supported on ARMv7 or later.  Especially, ARMv8
generation SoCs are likely to run ARM Trusted Firmware BL31.  In
this case, U-Boot is a non-secure world boot loader, so it should
not be able to reset the system directly.

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