i2c_nuttx:Use BOARD_NUMBER_I2C_BUSES and BOARD_I2C_BUS_CLOCK_INIT

Used the board overrideable vlaues to define the _bus_clocks
   array and initalize it.

src/drivers/device/i2c_nuttx.cpp

@@ -49,7 +49,7 @@ namespace device
  *  All calls to init() will NOT set the buss frequency
  */
 
-unsigned int I2C::_bus_clocks[3] = { 100000, 100000, 100000 };
+unsigned int I2C::_bus_clocks[BOARD_NUMBER_I2C_BUSES] = BOARD_I2C_BUS_CLOCK_INIT;
 
 I2C::I2C(const char *name,
 	 const char *devname,

src/drivers/device/i2c_nuttx.h

@@ -62,7 +62,7 @@ public:
 
 	static int	set_bus_clock(unsigned bus, unsigned clock_hz);
 
-	static unsigned	int	_bus_clocks[3];
+	static unsigned	int	_bus_clocks[BOARD_NUMBER_I2C_BUSES];
 
 protected:
 	/**