Revert "stm32: Change USART port to USART6 for stm32f746 discovery board"

Per Vikas' request, the problem this commit is supposed to be solving is
something he doesn't see and further this introduces additional hardware
requirements.

This reverts commit 4b2fd720a7b2f78c42d1565edf4c67f378c65440.

Signed-off-by: Tom Rini <trini@konsulko.com>
3 files changed