AP_HAL_ESP32: RCOutput: add support for brushed PWM mode

Also adds some infrastructure for changing PWM group mode, though this
is likely the complete set that can be supported with the ESP32 PWM
peripheral.

libraries/AP_HAL_ESP32/RCOutput.h

@@ -54,6 +54,8 @@ public:
     uint16_t read(uint8_t ch) override;
     void read(uint16_t* period_us, uint8_t len) override;
 
+    void set_output_mode(uint32_t mask, const enum output_mode mode) override;
+
     void cork() override;
     void push() override;
 
@@ -107,6 +109,7 @@ private:
 
         uint32_t rc_frequency; // frequency in Hz
         uint32_t ch_mask; // mask of channels in this group
+        enum output_mode current_mode; // output mode (none, normal, brushed)
     };
 
     struct pwm_chan {
@@ -121,6 +124,10 @@ private:
 
     uint32_t fast_channel_mask;
 
+    uint32_t constrain_freq(pwm_group &group);
+
+    void set_group_mode(pwm_group &group);
+
     void write_int(uint8_t chan, uint16_t period_us);
 
     static pwm_group pwm_group_list[];