AP_HAL: make hal aware of oneshot125

this separates oneshot and oneshot125 at the HAL layer, allowing the
HAL to properly handle the high timing resolution of oneshot125
(needed to use 125ns time steps, to get the full 1000 throttle steps)

libraries/AP_HAL/RCOutput.h

@@ -170,6 +170,7 @@ public:
         MODE_PWM_NONE,
         MODE_PWM_NORMAL,
         MODE_PWM_ONESHOT,
+        MODE_PWM_ONESHOT125,
         MODE_PWM_BRUSHED,
         MODE_PWM_DSHOT150,
         MODE_PWM_DSHOT300,