serial: stm32: rework wakeup management

Rework wakeup management by activating uart as wakeup source when usart
device OR its tty virtual device parent is wakeup source.

This patch aim to avoid potential misalignment between serial and tty
wakeup flags.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210319184253.5841-2-erwan.leray@foss.st.com


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