drivers: modem: Implement runtime power management for CMUX

CMUX driver can enable the support for idle-timer in
devicetree and can be requested to shut down the pipe
during sleep.

Then UART backend put the actual device into sleep when
pipe is closed.

Waking up is requested by sending data to DLC pipe
or by manually opening the uart_pipe.
Modem may request similar wake-up by a RING interrupt which
would open the same pipe.

When UART is powered and pipe is not closed, CMUX wake-up
procedure is automatic. Either end may initiate the wake-up.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>