[B96B_F446VE] change serial type

Use only the index, not the UARTName any more.
In case of app with 2 serial (using DMA) + 1 serial (stdio), we have found a bug. The dma handler is overwritten by the last initialized serial object.
Therefore read and write functions did not work anymore.
We have reworked this file to save 1 handler per UART IP, and align it with MBED OS file.
Tests have been passed. Same status as before (OK except MBED_37, manual test for SERIAL_ASYNC also OK).

Author: adustm

libraries/mbed/targets/hal/TARGET_STM/TARGET_STM32F4/TARGET_B96B_F446VE/objects.h

@@ -66,8 +66,7 @@ struct dac_s {
 };
 
 struct serial_s {
-    UARTName uart;
-    int index; // Used by irq
+    int index;
     uint32_t baudrate;
     uint32_t databits;
     uint32_t stopbits;