drivers: misc: devmux: a device multiplexer pseudo-device

The Device Multiplexer (devmux) is a pseudo-device that can
be used to select between multiple included sub-devices.

It is experimental, but its current use is in system
remediation. Take for example, the scenario where the
system console and log subsystem both have the uart backend
enabled. The case may arise, where the chosen backing uart
could be an abstraction of another very high-bandwidth bus
- such as a PCIe BAR, a UDP socket, or even even just memory.

If the "service" (for lack of a better term) that backs this
abstract "uart" experiences an error, it is of critical
importance to be able to switch the system console, uart log
backend, or whatever to another uart (semi-transparently) in
order to bring up a shell, continue to view system logs, or
even just support user console I/O.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>