Enabling servo usage on boards with PWM current control

Enabling servo usage on boards with PWM current control
timer5 is used by some boards like the Mini Rambo for controlling motor current via PWM, see stepper.cpp.
```cpp
  #ifdef MOTOR_CURRENT_PWM_XY_PIN
    pinMode(MOTOR_CURRENT_PWM_XY_PIN, OUTPUT);
    pinMode(MOTOR_CURRENT_PWM_Z_PIN, OUTPUT);
    pinMode(MOTOR_CURRENT_PWM_E_PIN, OUTPUT);
    digipot_current(0, motor_current_setting[0]);
    digipot_current(1, motor_current_setting[1]);
    digipot_current(2, motor_current_setting[2]);
    //Set timer5 to 31khz so the PWM of the motor power is as constant as possible. (removes a buzzing noise)
    TCCR5B = (TCCR5B & ~(_BV(CS50) | _BV(CS51) | _BV(CS52))) | _BV(CS50);
  #endif
}
```
Using the same timer for controlling servos results in loss of motor control. So use timer4/3 for those boards instead.
[See discussion here.](http://shop.prusa3d.com/forum/software-f13/enabling-auto-leveling-in-firmware-t416-s40.html)

Marlin/servo.h

@@ -60,12 +60,17 @@
 
 // Say which 16 bit timers can be used and in what order
 #if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
-  #define _useTimer5
   //#define _useTimer1
   #define _useTimer3
   #define _useTimer4
+  #ifndef MOTOR_CURRENT_PWM_XY_PIN
+    //Timer 5 is used for motor current PWM and can't be used for servos.
+    #define _useTimer5
     //typedef enum { _timer5, _timer1, _timer3, _timer4, _Nbr_16timers } timer16_Sequence_t ;
     typedef enum { _timer5, _timer3, _timer4, _Nbr_16timers } timer16_Sequence_t ;
+  #else
+    typedef enum {_timer3, _timer4, _Nbr_16timers } timer16_Sequence_t ;
+  #endif
 
 #elif defined(__AVR_ATmega32U4__)
   //#define _useTimer1