drivers/uart: stm32: Fix pm_constraint handling

Introduce new logic to set/release pm_constraint during serial TX
transactions.

First change is to introduce an internal flag and utility functions
to control the set/release constraint balancing per uart device.
This way, whatever the mix of transactions or API calls, we
ensure a single uart device can only do 1 or 0 to the PM state
constraint. Constraint can't then be set more than once, released w/o
having been set or released more than it was set.

The last part of the change reworks the triggers for constraints
set/release operations.
In order not to disturb driver operations, if irq driven mode or PM is
enabled, don't enable TC interrupt handling by default.
Instead, map the pm_constraint setting to the way TC flag is handled
in normal mode of operations (irq driven or async).
As a consequence, in irq driven mode, pm_constraint is set/released on
tx_enable/tx_disable api calls, which gives API user full control
on transaction protection vs low power operations.
Finally, we emulate the same behavior on TX poll transaction, by
enabling TC irq at the start of a stream and disabling TC irq once
stream is completed. This is controlled with a dedicated device flag.


Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>