msm: smd_tty: enable concurrent access to smd tty ports

smd_tty platform drivers are register at boot time which prevent the
access to DS port from kernel space for DUN mode.

To enable concurrent access register smd_tty platform drivers after
smd_tty_port_activate().

CRs-Fixed: 537817
Change-Id: I743678f1c71f67bacfdfabb4464907d0fc014d51
Signed-off-by: Arun Kuamr Neelakantam <aneela@codeaurora.org>