Arm: dts: stm32: remove extra compatible string for uart

This patch removes the extra compatibility string "st,stm32-usart" to
avoid confusion, save some time & space.

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>