core,xilinx,device: added option/methods to enable/disable quad mode on SPI Flash

src/xilinx.hpp

@@ -65,6 +65,10 @@ class Xilinx: public Device, SPIInterface {
 		 * \brief unprotect SPI flash blocks
 		 */
 		bool unprotect_flash() override;
+		/*!
+		 * \brief configure Quad mode for SPI Flash
+		 */
+		bool set_quad_bit(bool set_quad) override;
 		/*!
 		 * \brief erase SPI flash blocks
 		 */