clocksource: timer-riscv: Increase rating of clock_event_device for Sstc

When Sstc is available the RISC-V timer clock_event_device should be
the preferred clock_event_device hence we increase clock_event_device
rating for Sstc.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230710131902.1459180-3-apatel@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>