tty: smux_ctl: Add new SMUX port Support

smux_ctl exposes only one control port to userspace. Enabling of dual
PDN support requires two SMUX ports.

Add new SMUX control port to enable dual PDN support.

CRs-Fixed: 497343
Change-Id: I2a1d48f1e5d2acd1f67d7673d325c4360b783cf4
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>