ramips: 6.1: pwm: rewrite mtk_pwm_ops to fix pwm driver

ramips: 6.1: pwm: rewrite mtk_pwm_ops to fix pwm driver

Upstream dropped support for legacy driver [0]. Rewrite the driver like
the renesas pwm driver [1].

Fixes erros in the form of:
   make: *** [/__w/openwrt/openwrt/openwrt/include/toplevel.mk:232: target/compile] Error 1
  ====== Make errors from logs/target/linux/compile.txt ======
        |                   ^~~~~~~~~~~~~~
  drivers/pwm/pwm-mediatek-ramips.c:107:19: note: (near initialization for 'mtk_pwm_ops.free')
  drivers/pwm/pwm-mediatek-ramips.c:108:10: error: 'const struct pwm_ops' has no member named 'disable'
    108 |         .disable = mtk_pwm_disable,
        |          ^~~~~~~
  drivers/pwm/pwm-mediatek-ramips.c:108:20: error: initialization of 'int (*)(struct pwm_chip *, struct pwm_device *, struct pwm_capture *, long unsigned int)' from incompatible pointer type 'void (*)(struct pwm_chip *, struct pwm_device *)' [-Werror=incompatible-pointer-types]
    108 |         .disable = mtk_pwm_disable,
        |                    ^~~~~~~~~~~~~~~
  drivers/pwm/pwm-mediatek-ramips.c:108:20: note: (near initialization for 'mtk_pwm_ops.capture')
  cc1: all warnings being treated as errors

[0] - https://github.com/torvalds/linux/commit/0829c35dc5346e90f428de61896362b51ab58296
[1] - https://github.com/torvalds/linux/commit/ec00cd5e63f05461ab48128775c73c851c3c2b18

Signed-off-by: Nick Hainke <vincent@systemli.org>