soc: riscv: riscv-privileged: INTEL Nios V/g support

Add support for INTEL FPGA Nios V/g RISC-V based Processors.
Also amended SOC_NIOSV_M to use ATOMIC_OPERATIONS_BUILTIN.

Signed-off-by: Khor Swee Aun <swee.aun.khor@intel.com>