clocksource/drivers/timer-riscv: Clear timer interrupt on timer initialization

In the RISC-V specification, the stimecmp register doesn't have a default
value. To prevent the timer interrupt from being triggered during timer
initialization, clear the timer interrupt by writing stimecmp with a
maximum value.

Fixes: 9f7a8ff6 ("RISC-V: Prefer sstc extension if available")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ley Foon Tan <leyfoon.tan@starfivetech.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240306172330.255844-1-leyfoon.tan@starfivetech.com