lantiq: fix timer driver build warnings

lantiq: fix timer driver build warnings

This patch fixes the following build warnings:

arch/mips/lantiq/xway/timer.c:160:14: error: no previous prototype for 'ltq_get_fpi_bus_clock' [-Werror=missing-prototypes]
  160 | unsigned int ltq_get_fpi_bus_clock(int fpi) {
      |              ^~~~~~~~~~~~~~~~~~~~~
arch/mips/lantiq/xway/timer.c:803:36: error: invalid use of undefined type 'struct platform_device'
  803 |         ret = devm_mutex_init(&pdev->dev, &timer_dev.gptu_mutex);
      |                                    ^~
arch/mips/lantiq/xway/timer.c:864:12: error: no previous prototype for 'lq_gptu_init' [-Werror=missing-prototypes]
  864 | int __init lq_gptu_init(void)
      |            ^~~~~~~~~~~~
arch/mips/lantiq/xway/timer.c:873:13: error: no previous prototype for 'lq_gptu_exit' [-Werror=missing-prototypes]
  873 | void __exit lq_gptu_exit(void)
      |             ^~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>