Add a note for enabling wakeup capabilities of usart
Signed-off-by: NBich Hemon <bich.hemon@st.com>
Signed-off-by: NErwan Le Ray <erwan.leray@st.com>
Reviewed-by: NRob Herring <robh@kernel.org>

Link: https://lore.kernel.org/r/1560433800-12255-2-git-send-email-erwan.leray@st.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>