[STM32F4] SPI ASYNCH support

This commit is highly derived from an existing work from salkinium.

It adds SPI ASYNC support to STM32 F4 devices. This required a small
rework of the structure to have SYNCH and ASYNCH co-exist.

Author: Laurent MEUNIER

hal/targets/hal/TARGET_STM/TARGET_STM32F4/common_objects.h

@@ -69,6 +69,8 @@ struct serial_s {
 };
 
 struct spi_s {
+    SPI_HandleTypeDef handle;
+    IRQn_Type spiIRQ;
     SPIName spi;
     uint32_t bits;
     uint32_t cpol;
@@ -80,6 +82,11 @@ struct spi_s {
     PinName pin_mosi;
     PinName pin_sclk;
     PinName pin_ssel;
+#ifdef DEVICE_SPI_ASYNCH
+    uint32_t event;
+    uint8_t module;
+    uint8_t transfer_type;
+#endif
 };
 
 #include "gpio_object.h"