hw/rtc/ls7a_rtc: Fix timer call back function

Replace qemu_irq_pulse with qemu_irq_raise in ls7a_timer_cb function
to keep consistent with hardware behavior when raise irq.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220701093407.2150607-3-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>