hw/timer/xilinx_timer: Use XpsTimerState instead of 'struct timerblock'

hw/timer/xilinx_timer: Use XpsTimerState instead of 'struct timerblock'

This remove a use of 'struct' in the DECLARE_INSTANCE_CHECKER()
macro call, to avoid after a QOM refactor:

  hw/timer/xilinx_timer.c:65:1: error: declaration of anonymous struct must be a definition
  DECLARE_INSTANCE_CHECKER(struct timerblock, XILINX_TIMER,
                           ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar@zeroasic.com>
Message-id: 20230109140306.23161-15-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>