arm: socfpga: soc64: Override 'lowlevel_init' to support ATF

Override 'lowlevel_init' to make sure secondary CPUs trapped
in ATF instead of SPL. After ATF is initialized, it will signal
the secondary CPUs to jump from SPL to ATF waiting to be 'activated'
by Linux OS via PSCI call.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>