soc: stm32f7: Fix fixup file for uart devices

UART related defines in STM32F7 files where filled with
references to USART.
Instances 4, 5, 7 and 8 of SoC serial port are actually UARTs.
So rename define's accordingly. Otherwise it couldn't build.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>