tty: smux_ctl: Add support for poll method.

Userspace clients hold a wakelock while calling read.  If data isn't
available, read() blocks preventing the release of the wakelock and
preventing the system from going into low power mode.

Implement poll() method to enable clients to call poll() to block and
wait for data without a wakelock.  Once poll() returns, then a wakelock
can be acquired and read() called without blocking which allows
minimizing the time a wakelock is held.

CRs-fixed: 379519
Change-Id: Icea23783ee2d03e2beac81bdb1b735e4edfd1d22
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>