clocksource/drivers/riscv_timer: Provide the sched_clock

Currently, we don't have a sched_clock registered for RISC-V systems.
This means Linux time keeping will use jiffies (running at HZ) as the
default sched_clock.

To avoid this, we explicity provide sched_clock using RISC-V rdtime
instruction (similar to riscv_timer clocksource).

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>