arm: socfpga: Add secure register access helper functions for SoC 64bits

These secure register access functions allow U-Boot proper running
at EL2 (non-secure) to access System Manager's secure registers
by calling the ATF's PSCI runtime services (EL3/secure).

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>