sysreset: socfpga: soc64: Enable L2 reset

sysreset: socfpga: soc64: Enable L2 reset

Put all slave CPUs (CPU1-3) into WFI mode. Master CPU (CPU0) writes
the magic word into system manager's scratch register to indicate
the system has performed L2 reset and request reset manager to
perform hardware handshake and then trigger L2 reset. CPU0 put
itself into WFI mode. L2 reset will reboot all HPS CPU cores after
which all HPS cores are in WFI mode. L2 reset is followed by warm
reset request by SPL via RMR_EL3 system register.

Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>