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 <patrice.chotard@st.com>
Reviewed-by: Christophe KERELLO <christophe.kerello@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>