binding and generic support for external uart supply

Hi,

I have a board with a couple of uarts routed to external connectors,
through a galvanic isolation. In order to actually use those uarts, I
need to turn on a regulator (in my schematic called 5V0_PERI), which I
can easily describe as a fixed-regulator with an enable gpio.

However, there doesn't seem to be any generic "ext-supply" binding or
similar for uarts; I would think this should be in
Documentation/devicetree/bindings/serial/serial.yaml if it did exist.

So the first question is: Can we add a binding for such an optional
ext-supply?

Assuming that's uncontroversial, the next question is on the driver (or
rather, as this is quite generic, serial core) side: how and where do we
enable that regulator? There's an uart_change_pm() which it would seem
natural to hook into, but that returns void, whereas a
regulator_enable() can return an error which I suppose we'd need to
propagate. Then there's uart_port_startup() which sounds promising
("called once per open"), but then I can't figure out where to put the
corresponding regulator_disable().

Thanks,
Rasmus