hw/rtc/ls7a_rtc: Use tm struct pointer as arguments in toy_time_to_val()

Use pointer as arguments in toy_time_to_val() instead of struct tm.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220701093407.2150607-7-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>