msm: smd_tty: Support multiple unique smd channels with the same name

When the Linux SMD driver detects a new channel, it creates a platform
device using the name of the channel as the name of the device, and uses
the channel type as the instance id in the platform device. Platform
drivers can only list the name of the device that the driver supports,
and only one platform driver is allowed to be registered for a given
device name.

The smd_tty port activation is failing for the smd channels which has same
name and tries to register same platform drivers again.

Add platfrom drivers registration check to avoid the multiple
registration of same platform drivers.

Change-Id: Iedf78e6931ac1982a203512e2a78c5939c487296
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>