serial: stm32x7: align compatible with kernel one

serial: stm32x7: align compatible with kernel one

stm32x7.c driver is dedicated for STM32F7.
In kernel, "st,stm32-usart" and "st,stm32-uart" compatible
strings are dedicated for STM32F4.

To keep U-boot and kernel aligned, replace the serial compatible
string from "st,stm32-usart", "st,stm32-uart" to
"st,stm32f7-usart", "st,stm32f7-uart" specific for STM32F7.

Signed-off-by: Patrice Chotard <[email protected]>
Reviewed-by: Christophe KERELLO <[email protected]>
Reviewed-by: Patrick DELAUNAY <[email protected]>
Acked-by: Vikas MANOCHA <[email protected]>