Arm: dts: stm32: remove extra compatible string from DT & driver

This patch remove the extra compatibility string "st,stm32-usart" from
driver & device tree.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>