tinyusb: Set CFG_TUSB_MCU for STM32F7
TinyUSB requires definition of CFG_TUSB_MCU.
For STM32F7 it was not defined at all.
Now it is defined and targets for STM32F7 with USB enabled
can be used.
#if MYNEWT_VAL(MCU_STM32F4)
#define CFG_TUSB_MCU OPT_MCU_STM32F4
+#elif MYNEWT_VAL(MCU_STM32F7)
+#define CFG_TUSB_MCU OPT_MCU_STM32F7
#elif MYNEWT_VAL(MCU_STM32L4)
#define CFG_TUSB_MCU OPT_MCU_STM32L4
#endif
