clocksource/drivers/riscv: Make RISCV_TIMER depends on RISCV_SBI

The riscv timer is set via SBI timer call, let's make RISCV_TIMER
depends on RISCV_SBI, and it also fixes some build issue.

Fixes: d5be89a8

 ("RISC-V: Resurrect the MMIO timer implementation for M-mode systems")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201028131230.72907-1-wangkefeng.wang@huawei.com