riscv: Make A ISA extension selectable

riscv: Make A ISA extension selectable

Make the Atomic (A) ISA extension selectable. Thus CPUs such as
Ibex without the A extension can be supported.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>