serial: stm32: add pm_runtime support

Use pm_runtime for clock management.

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>

Link: https://lore.kernel.org/r/1560433800-12255-5-git-send-email-erwan.leray@st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>