RISCV_TIMER/CLINT_TIMER is required for RISC-V system, and it
provides sched_clock, which allow us to enable IRQ_TIME_ACCOUNTING.
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>