ARM: dts: stm32: use stm32h7 usart compatible string for stm32h743

Previously the FIFO on the stm32h743 usart was not utilized, because
the stm32f7 compatible configures it without FIFO support.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>