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>

arch/riscv/Kconfig

@@ -24,6 +24,7 @@ config RISCV
 	select GENERIC_CPU_DEVICES
 	select GENERIC_IRQ_SHOW
 	select GENERIC_PCI_IOMAP
+	select GENERIC_SCHED_CLOCK
 	select GENERIC_STRNCPY_FROM_USER
 	select GENERIC_STRNLEN_USER
 	select GENERIC_SMP_IDLE_THREAD