AP_HAL: added clock_pulse() method to SPIDevice

this is used by SPI SDCard interface for HAL_ChibiOS

libraries/AP_HAL/SPIDevice.h

@@ -45,6 +45,12 @@ public:
     virtual bool transfer_fullduplex(const uint8_t *send, uint8_t *recv,
                                      uint32_t len) = 0;
 
+    /* 
+     *  send N bytes of clock pulses without taking CS. This is used
+     *  when initialising microSD interfaces over SPI
+    */
+    virtual bool clock_pulse(uint32_t len) { return false; }
+    
     /* See Device::get_semaphore() */
     virtual Semaphore *get_semaphore() override = 0;