stm32: Change USART port to USART6 for stm32f746 discovery board

This change is to remove a halt at about 200KiB
while sending a large(1MiB) binary to a micro controller using USART1.
USART1 is connected to a PC via an on-board ST-Link debugger
that also functions as a USB-Serial converter.
However, it seems to loss some data occasionally.
So I changed the serial port to USART6 and connected it to the PC using
an FTDI USB-Serial cable, therefore the transmission was successfully
completed.

Signed-off-by: Toshifumi NISHINAGA <tnishinaga.dev@gmail.com>