arm: socfpga: misc: Add CONFIG_SYS_L2_PL310 switch

Preparation for Stratix 10 enablement. In ARM64, L2 cache controller is
accessed through processor registers. So, add CONFIG_SYS_L2_PL310 switch
conditional build in order this file can by shared across other SOCFPGAs.

Signed-off-by: Chin Liang See <chin.liang.see@intel.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>