RISC-V: Select GENERIC_SCHED_CLOCK for clocksource drivers

The riscv_timer driver can provide sched_clock using "rdtime"
instruction but to achieve this we require generic sched_clock
framework hence this patch selects GENERIC_SCHED_CLOCK for RISCV.

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