Add baseAddr function to HwPWM instance

This allows for PWM configuration when direct register access is required.

Author: jgartrel

cores/nRF5/HardwarePWM.h

@@ -68,6 +68,12 @@ class HardwarePWM
 
     void setClockDiv(uint8_t div);      // value is PWM_PRESCALER_PRESCALER_DIV_x, DIV1 is 16Mhz
 
+    // Direct access to peripheral base address
+    NRF_PWM_Type * const baseAddr(void)
+    {
+      return _pwm;
+    }
+
     // Cooperative ownership sharing
 
     // returns true ONLY when (1) no PWM channel has a pin, and (2) the owner token is nullptr